BuUetin  No.  209 


Series  t  ^'  descriptive  Geology, 
I D,  Petrography  and  Mi 


Mineralogy,  22 


DEPARTMENT  OF  THE  INTERIOR 

UNITED  STATES  GEOLOGICAL  SURVEY 

CHARLES  D.  WALCOTT,  Director 


THE 


BY 


REGINALD   ALDWORTH   DALY 


WASHINGTON 

GOVERNMENT     PRINTING     OFFICE 
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CONTENTS. 


Page. 

Introduction 7 

Nature  of  the  investigation 7 

Acknowledgments 7 

Chapter  I.  Physical  geography 8 

General  topography  of  the  area 8 

Drainage 10 

Glaciation  of  Mount  Ascutney 13 

Summary 13 

Chapter  II.  General  description  of  the  schists  in  the  area 14 

Phyllitic  series 14 

Gneissic  series 17 

Geologic  age  of  the  schists  and  of  the  intrusive  rocks 19 

Summary „._  20 

Chapter  III.  Contact  metamorphism 22 

The  metamorphic  aureole . 22 

Changes  in  limestones 23 

Metamorphism  of  the  phyllites 24 

Series  A  of  specimens  from  the  metamorphic  aureole 24 

Series  B  of  specimens  from  the  metamorphic  aureole 31 

Series  C  of  specimens  from  the  metamorphic  aureole 33 

Comparison  of  the  metamorphic  effects 33 

Summary ^ 35 

Chapter  IV.  The  eruptive  rocks .  36 

General  table  and  correlation ^  36 

Gabbros,  diorites,  and  related  rocks : 38 

Basic  stock ;  its  gabbro  and  diorite  phases 38 

•   Basic  segregations 43 

Dioritic  dikes  cutting  the  Basic  stock 44 

' '  Windsorite ' '  dikes  cutting  the  Basic  stock 45 

Syenites 48 

Main  syenite  stock;  its  phases 48 

Basic  segregations . . 64 

Endomorphic  zone  of  the  syenite  stock 68 

Great  syenite-porphyry  dike  of  Little  Ascutney  Mountain 69 

Syenite  stock  of  Pierson  Peak 70 

Aplitic  dikes  cutting  the  syenites 70 

Paisanite  dike  cutting  the  Main  stock i 70 

Basic  segregations 72 

Common  muscovite-aplite  of  the  Main  stock 73 

Paisanite  dike  on  Little  Ascutney  Mountain 73 

Breccia  masses  on  Little  Ascutney  Mountain 77 

Biotite-granite  stock 79 

Basic  segregations  in  the  granite 82 

Endomorphic  zone  of  the  granite 84 

3 


4  CONTENTS. 

Chapter  IV — Contintied.  Page. 

Lampropliyres 85 

Camptonites 86 

Diabase  dikes 87 

Summary v 88 

Chapter  V.  Theoretical  conclusions 90 

Manner  of  intrusion  of  the  stocks 90 

Application  of  existing  theories  to  the  Asctitney  intrusions 91 

Suggested  hypothesis  of  the  manner  of  intrusion 93 

Abyssal  assimilation 100 

Evidences  of  differentiation 104 

The  petrogenic  cycle 107 

Stimmary  and  general  application 108 

The  universal  earth  magma 110 

Appendix.  Tables,  list  of  specimens,  etc 115 

Table  showing  mineralogical  and  structural  constitution  of  the  Ascut- 

ney  eruptives 115 

Table  of  chemical  analyses 118 

List  of  the  more  iraportant  specimens  studied . 130 

Index 121 


ILLUSTRATIONS. 


Page. 
Plate  I.  A,  General  view  of  Asciitney  Mountain,  Pierson  Peak,  and  Little 
Asctitney  Mountain;  B,  Ascutney  Mountain  and  the  terraces 

of  the  Connecticut  River 7 

II.  A,  Unaltered  phyllite,  showing  normal  plane-parallel  strvicture; 

B,  Phyllite,  showing  bent  laminae  and  strain-slip  cleavage 16 

III.  A,  Hornfels  containing  abundant  pleonaste  and  cornnduni  in 

a  matrix  composed  chiefly  of  cordierite;  B,  Thin  section  of 
quartz-bearing  hornblende-biotite-diorite,  showing  an  apatite 
crystal  inclosing  a  core  of  brown  glass 32 

IV.  A,  Typical  thin  section  of  the  nordmarkite  of  the  Main  stock, 

granitic  phase;  B,  Pyroclastic  feldspar  (soda-orthoclase)  siir- 
rounded  by  a  reaction  rim  rich  in  alkaline  hornblende ,  from 
the   'large  paisanite  dike    on    northwest    slope    of    Ascutney 

Mountain . .         58 

V.  A,  Segregation  of  mica  and  hornblende  concentrically  arranged, 
in  paisanite;  B,  Basic  segregations  in  nordmarkite  at  Crystal 

Cascade 62 

VI.  Basic  segregations  and  inclxisions  of  schist  in  nordmarkite   at 

Crystal  Cascade 64 

VII.  Geologic  map  and  section  of  Ascutney  Mountain  and  vicinity 70 

Pig.  1.  Sketch  x^lan  of  intrusive  rocks  on  Little  Ascutney  Mountain 74 

5 


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http://www.archive.org/details/geologyofascutneOOdaly 


U.    S.    GEOLOGICAL  SURVE 


BULLETIN    NO.    209       PL 


GENERAL   VIEW    OF    ASCUTNEY    MOUNTAIN,    PIERbON    PEAK,    AND    LITTLE 
ASCUTNEY    MOUNTAIN. 
From  a  point  on  the  New  Hampshire  side  of  the  Connecticut  River,  looking  northwest. 


.      '-Uii:Lr'',,    ,  AND    THE   TERRACES   OF   THE   CONNECTICUT    RIVER. 

Ascutneyville  in  the  mid-distance,  looking  northwest  across  the  Connecticut  River  terraces. 


THE  GEOLOGY  OF  ASCUTNEY  MOUNTAIN,  VERMONT. 


By  R.  A.  Daly. 


INTRODUCTION. 

Nature  of  the  investigation. — The  following  pages  embody  the  results 
of  an  investigation  of  the  lithology  and  geology  of  a  plexus  of  eruptive 
rocks  and  of  the  metamorphic  aureole  in  schistose  rocks  surrounding 
the  igneous  bodies.  The  field  Avork  was  begun  in  1893,  but  numerous 
interruptions  prevented  the  completion  of  the  study  until  the  present 
year.  In  the  meantime  an  elaborate  series  of  chemical  analyses  was 
made  by  Dr.  Hillebrand  (in  1896)  and  the  results  were  published  on 
pages  68-70  of  Bulletin  148  of  the  United  States  Geological  Survey. 
These  analyses  are  here  republished,  with  the  oxides  arranged  in  the 
order  recommended  by  Dr.  H.  S.  Washington. « 

Acknowledgments. — The  writer's  best  thanks  are  due  to  Dr.  Hille- 
brand, for  the  completeness  and  accuracy  of  his  analyses;  to  Prof. 
J.  E.  Wolff,  of  Harvard  University,  who  not  only  suggested  this  piece 
of  research,  but  also  greatly  assisted  in  the  petrographical  determina- 
tions ;  to  Professor  Rosenbusch,  of  Heidelberg,  who  likewise  aided  in 
the  laboratory  study  of  the  collected  material;  to  Dr.  F.  P.  Gulliver, 
for  the  care  he  bestowed  on  the  preparation  of  the  topographical  map; 
and  especially  to  Dr.  T.  A.  Jaggar,  of  Harvard  University,  who,  after 
carrying  on  several  weeks'  field  work  in  the  area  in  collaboration, 
placed  his  notes  and  rock  collection  at  the  disjposal  of  the  writer.  In 
addition,  Dr.  Jaggar  has  done  much  in  the  microscopic  investigation 
of  the  specimens  and  in  preparing  the  photographic  illustrations  for 
this  repoi't.  He  has  also  read  the  manuscript,  which  has  been 
improved  both  in  form  and  contents  by  his  valuable  suggestions- 

aAm.  Jour.  Sci.,  4th  series,  Vol.  X,  1900,  p.  59, 


CHAPTER  L 
PHYSICAL   GEOGRAPHY. 
GENERAL  TOPOGRAPHY   OF  THE  AREA. 

Mount  Aseiitney  is  the  most  conspicuous  elevation  seen  by  the 
traveler  in  ascending  the  Connecticut  River  (see  PL  I,  A).  The  moun- 
tain, as  well  as  the  rest  of  the  area  considered  in  this  paper,  is  sit- 
uated on  the  right  bank  of  the  river  and  near  the  town  of  Windsor,  in 
southeastern  Vermont.  Though  having  an  elevation  of  little  more 
than  3,000  feet  (915  meters)  above  sea  level,  Ascutney  is  very  prom- 
inent as  it  rises  from  the  floor  of  the  deeply  trenched  master  valley  of 
New  England.  The  railway  bridge  over  the  Connecticut  at  Windsor 
is  but  301  feet  (92  meters)  above  sea  level;  the  summit  of  the  moun- 
tain is,  according  to  Dr.  Gulliver's  determination,  3,114  feet  (950 
meters)  above  the  same  datum «  and  lies  only  3  miles  from  the  river 
(see  PI.  I,  B).  Thus  the  mountain  is  considerably  more  imposing  than 
many  other  peaks  in  New  England,  wliich,  although  of  the  same  or 
even  greater  height,  yet  rise  from  a  more  elevated  base.  Additional 
scenic  importance  attaches  to  Ascutney  on  account  of  its  isolated 
position.  Among  the  nearer  noteworthy  elevations  are  Ludlow  and 
Shrewsbury  mountains  and  Killington  Peaks  of  the  Green  Mountain 
Range;  accordingly,  for  a  distance  of  20  miles  in  every  direction, 
the  beautifully  compact,  broadly  conical  outline  of  Ascutney  forms  a 
principal  feature  of  the  landscape.  Largely  for  this  reason  the 
mountain  enjoys  a  special  reputation  for  beauty  among  the  inhab- 
itants and  tourists  of  New  England. 

The  conditions  for  field  work  are  good  except  in  some  parts  of  the 
main  mountain,  where  thick  second-growth  timber  effectually  conceals 
the  eruptive  rocks.  Two  good  paths  to  the  summit,  one  from  Browns- 
ville, the  other  from  the  Windsor  side,  were  open  in  1898.  The  moun- 
tain can,  however,  be  easily  climbed  from  any  direction. 

The  softened  j)rofiles  of  the  mountain  suggest,  and  a  study  of  the 
geological  structure  of  the  region  proves,  that  Ascutney  is  a  residual 
of  erosion  (see  PI.  VII).  It  has  been  carved  out  of  this  part  of  the  once 
lofty  Appalachian  mountain  system  where  the  sedimentary  rocks  of 
the  range  have  been  intruded  by  several  stocks  and  thick  dikes  of 
igneous  rock.  The  relief  features  of  the  area  discussed  thus  belong 
to  the  same  categorj^  as  the  very  common  sugarloaf  peaks  of  Vermont, 

«C.  H.  Hitchcock  estimated  the  height  to  be  "about  3,1G8  feet:  "  Geology  of  New  Hampshire, 
Vol.  I,  1877,  p.  180. 


DALY.J  TOPOGRAPHY.  9 

located  on  intrusive  granites  and  syenites.  The  geological  map 
(PL  VII)  indicates  in  an  almost  diagrammatic  fashion  the  sympathy 
between  relief  and  rock  composition.  Ascntney  itself  owes  its  exist- 
ence primarily  to  a  great  stock  of  qnartz-syenite.  The  picturesque 
ridge  of  Little  Ascntney  is  held  up  by  a  strong  rib  of  intrusive  syenite- 
porphyry  associated  with  other  eruptives  more  resistant  to  the  weather 
than  either  the  gabbro-diorite  stock  on  the  north  or  the  gneisses  on 
the  south.  The  shapely  cone  north  of  Little  Ascntney,  which,  for 
purposes  of  convenience  in  this  report,  has  been  named  "  Pierson 
Peak "  (after  the  hospitable  owner  of  the  farm  at  the  base  of  the 
hill),  is  strictly  controlled  in  form  by  a  small  elliptical  stock  of  alka- 
line syenite  cutting  the  softer  diorites. 

Apart  from  other  more  general  considerations,  the  fact  that  these 
eruptive  rocks  are  more  resistant  to  weather  than  the  surrounding 
schists  is  clear  from  the  nature  of  the  slopes  and  profiles  at  the  con- 
tacts. As  a  rule  there  is  an  abrupt  steepening  of  the  ascent  along 
the  radiating  spurs  of  the  main  mountain  just  above  the  contact 
between  the  sedimentary  and  eruptive  rocks.  At  the  same  line  of 
contact  there  is  likewise  a  sudden  change  of  gradient  in  the  streams 
draining  the  mountain,  as  if  corrasion  were  considerably  easier  oa 
the  schists  than  on  the  eruptive  rocks.  An  example  is  seen  at  the 
beautiful  "Crystal  Cascade"  on  the  southwest  side  of  the  main  moun- 
tain. This  general  feature  of  the  residuals  of  erosion  in  our  area  may 
be  repeatedly  and  clearly  seen  in  the  granitic  hills  of  New  England 
and  might  serve  to  dispel  anj^  doubts  which  remain  in  the  minds  of 
those  students  of  erosion  who  are  skeptical  as  to  the  prevailing  theory 
of  New  England  reliefs,  for  it  is  doubtless  true  that  the  most  illumi- 
nating treatment  of  New  England  topography  finds  best  explanatian 
for  its  mountains  and  higher  hills  in  the  assumption  of  tlie  supe- 
rior strength  of  their  component  rocks — a  strength,  namely,  superior 
to  that  of  the  rock  masses  immediately  surrounding.  In  a  score  or 
two  of  instances  in  Vermont  and  New  Hampshire  striking  differences 
of  relief  are  faithfully  associated  with  equally  striking  differences  of 
lithologic  composition.  Plutonic  eruptives  compose  the  mountains 
and  generally  weak  schists  underlie  the  encircling  lowlands.  In  these 
examples  the  greater  height  of  the  hills  can  scarcely  be  due  to  more 
pronounced  initial  uplift  during  the  original  mountain  building. 
When,  on  the  other  hand,  differential  erosion  is  so  clear  for  granitic 
mountains  like  Ascntney,  it  seems  legitimate  to  extend  the  idea  to 
many  New  England  residuals  of  schistose  composition,  where,  as  yet, 
full  corroborative  evidence  as  to  the  validity  of  the  same  assumption 
is  not  obtainable. 

The  view  that  the  rocks  of  the  classic  monadnock  in  New  Hamp- 
shire are  harder  than  the  somewhat  similar  rocks  about  that  moun- 
tain certainly  wins  most  credibility  from  the  general  agreement  of 
that  assumption  with  the  most  fruitful  explanation  of  the  New  England 
peneplain ;  but  it  would  be  a  matter  of  considerable  satisfaction  to 


10  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull. 209. 

those  who  have  to  deal  in  theories  of  land  sculpture  if  detailed 
petrographic  and  field  studies  of  the  schist  monadnocks  were  made 
with  intent  to  test  the  theory  of  differential  hardness.  Perhaps  a 
microscopic  study  of  similarly  exposed  monadnock  rocks  and  low- 
land rocks  would  enable  the  investigator  to  ascertain  the  amount  of 
post-Glacial  weathering  which  has  occurred.  If  quantitative  estimates 
were  likewise  made  as  to  the  influence  of  jointage,  rifting,  cleavage, 
etc.,  the  result  should  be  to  give  a  more  scientific  basis  for  the  dis- 
cussion of  the  residuals  of  erosion  than  now  exists.  In  a  rough  way 
an  analogous  but  incomplete  study  of  the  rocks  of  this  area  has  sus- 
tained the  monadnock  theory  of  the  origin  of  Ascutney  and  Little 
Ascutney.  This  theory,  to  be  sure,  here  scarcelj^  needs  other  substan- 
tiation than  the  facts  of  composition,  structure,  and  present  relief. 

DEAII^AGE. 

The  CoEinecticut  River  flows  along  a  belt  of  soft  rocks  parallel  to 
their  strike,  and  is  thus  a  typical  longitudinal  valley.  In  no  psbrt  of 
its  course  is  it  more  clearly  "adjusted"  to  a  relatively  weak  zone  than 
on  the  '' Oalciferous  mica-schist"  eastward  of  the  mountain.  Simi- 
larly Mill  Brook  follows  the  strike  of  the  rocks  in  that  gorge-like  part 
of  its  course  between  the  elbow  south-southeast  of  Windsor  and  its 
confluence  with  the  Connecticut.  Elsewhere  Mill  Brook,  like  the 
stream  entering  the  main  river  near  Ascutney ville,  belongs  to  the 
class  of  "superposed"  streams,  having  sunk  its  channel  irregularly 
through  drift  and  terrace  sands  into  the  underlying  schists.  Short 
but  broad  valleys,  located  partly  on  schists,  partly  on  the  compara- 
tively soft  diorite,  separate  Little  Ascutney  and  Ascutney  Mountain 
proper.  These  valleys  are  also  adjusted  to  weak  zones  in  the  rocks, 
and  belong  to  the  now  well-recognized  class  of  ' '  subsequent "  valleys. 

But  it  is  not  eas}^  to  place  the  radiating  drainage  of  the  main  moun- 
tain in  the  accepted  classification  of  stream  courses.  There  is  nothing 
to  show  that  the  eruptives  of  the  area  ever  reached  the  surface  to  form 
volcanic  flows  or  cones;  they  seem  rather  to  have  consolidated  in  the 
form  of  a  complex  stock-like  boss.  The  structure  of  the  region  shows 
that  the  radiating  drainage  is  not  the  result  of  inheritance  from  the 
surface  of  a  dome  in  the  overlying  schists,  in  which  a  different  pattern 
of  drainage  would  have  predominated,  namely,  a  more  or  less  rectan- 
gular network  of  stream  courses.  Such  a  dome  would  not  likely  be 
able  to  alter  seriously  the  directions  of  the  streams  originating  either 
in  the  folding  of  the  schists  or  in  the  process  by  which  newer  valleys 
would  be  worn  out  on  weak  belts  parallel  to  the  strike.  These  radi- 
ating streams  can  not,  thus,  be  regarded  as  "superposed"  through 
the  schist  blanket  once  overlying  the  stocks. 

There  is  here,  in  fact,  a  kind  of  drainage  which  is  controlled  in  its 
development  by  constructional  processes  fundamentally  different  from 
those  usually  considered  in  a  systematic  discussion  of  streams.  Fold- 
ing, faulting,  and  glacial  and  volcanic  accumulation  are  examples  of 


DALY.]  DEAINAGE.  11 

processes  leading  to  the  formation  of  surfaces  which  are,  in  the  initial 
stage,  exposed  to  erosion.  But  there  is  a  kind  of  subterranean  con- 
struction to  be  found  in  the  intrusion  of  large  bodies  of  igneous  rock, 
which  may,  in  the  course  of  time,  affect  the  relief  and  drainage  of  a 
region  much  more  conspicuously  than  the  processes  just  mentioned. 
The  uncovering,  by  erosion,  of  a  boss  of  igneous  material  harder  than 
the  surrounding  rock  formations  will  necessitate  either  the  true 
"superposition"  of  streams  in  the  manner  just  suggested,  but  excluded, 
for  good  reasons,  in  the  Ascutne}^  instance,  or  the  formation  of  new 
ones  divergent,  roughly  speaking,  from  the  center  of  the  boss  toward 
the  lowlands  of  the  l6ss  resistant  formations.  These  latter  streams 
are  logically  consequent  on  the  intrusion  and,  to  a  greater  or  less 
extent,  consequent  in  length  and  direction  on  the  original  contours 
and  ground  plan  of  the  irruptive  body.  This  may  be  true  in  a  large 
way  whatever  the  details  of  form  in  the  upper  surface  of  the  igneous 
mass.  Whether  it  be  a  regular  boss  with  smooth  profiles,  or  one 
irregularly  terminated  by  apophyses  into  the  overlying  rock,  the 
superior  hardness  of  the  intrusive  will,  in  the  end,  tend  to  cause  its 
projection,  as  a  whole,  above  the  soft-rock  terrane;  so  that  there  will 
be  brought  about  an  approximation  to  the  average  original  profile  of 
the  boss.  There  must  in  any  case  originate  on  its  revealed  surface 
a  number  of  streams  divergent  from  the  central  region  of  the  boss 
and  fiowing  toward  the  surrounding  lower  land. 

Such  streams  are  seen  to  be  analogous  to  those  which  drain  the 
retreating  escarpments  of  tilted  stratified  beds — the  class  of  "  obse- 
quent "  streams  as  defined  by  Professor  Davis.  Obsequent  streams 
drain  the  scarped  front  of  a  hard  member  of  the  stratified  series  and. 
are  the  result  of  the  excavation  of  lowlands  by  the  lengthening  and 
widening  of  valleys  in  an  underlying  softer  formation.  The  radial 
drainage  here  considered  is  similarly  caused  by  the  removal  of  rock 
material  less  resistant  to  the  weather  than  the  intrusive  igneous  rock. 
At  the  same  time,  that  removal  means  the  origination  of  drainage 
"  adjusted"  to  the  soft  encircling  formation.  The  adjustment  is  here 
circumferential  and  centrifugal  with  reference  to  the  middle  point  of 
the  intrusive  body,  not  longitudinal  (parallel  to  the  strike  of  a  bedded 
formation),  as  in  the  case  of  those  "  subsequent"  streams  into  which 
"  obsequents  "  pour  their  waters. 

The  radial  drainage  of  Ascutney  is  thus  believed  to  owe  its  origin 
to  the  degradation  of  the  encircling  schists — a  centrifugal  control 
due  to  differential  hardness.  Located  on  a  hard  member,  they  are 
to  be  associated  with  obsequent  drainage,  and  share  with  obsequent 
and  subsequent  drainage  the  characteristic  of  appearing  only  rela- 
tively late  in  the  whole  geographical  cycle  of  degradation.  They  are 
also  conditioned  by  the  original  form  of  the  intrusive,  and  are  thus 
consequent.  To  express  their  composite  nature  they  may  be  called 
suhconsequent,  using  a  term  which  was  first  proposed  by  Professor 
Davis  for  what  are  generally  coming  to  be  called   "subsequent" 


12      ■  GEOLOGY    OF   ASCUTKEY   MOUNTAIN,  VERMONT.         [bull.  209. 

streams.  His  abandonment  of  the  longer  for  the  shorter  term,  which 
was  independently  invented  by  him  and  by  Jukes,  leaves  "subcon- 
sequent "  open  to  the  special  use  to  which  we  propose  to  attach  it. 
The  prefix  "sub"  is  especially  appropriate,  as  it  serves  to  indicate 
the  necessary  lack  of  absolute  and  exact  control  possessed  by  the 
constructional  form  of  the  intruded  body  over  the  trend  of  this  class 
of  streams  even  where  they  run  over  the  igneous  rock.  That  control 
will,  to  some  extent,  be  imperfect  on  account  of  a  variety  of  circum- 
stances connected  with  the  removal  of  the  cover  and  the  apophyses 
penetrating  the  cover.  While  subconsequent  drainage  is  always 
divergent,  it  may  be  radial  or  elliptical  where  the  intrusive  has  a 
circular  or  elliptical  ground  plan;  or  bilateral,  as  in  the  case  of 
many  batholiths  and  great  dike-like  intrusions;  or,  finally,  irregularly 
divergent. 

GLACIATION  OF  MOUNT  ASCTJTNEY. 

The  similarity  of  form  between  Mount  Ascutney  and  other  residuals 
in  the  glaciated  tract  of  New  England,  on  the  one  hand,  and  the  residuals 
of  Georgia  and  the  Carolinas  on  the  other,  particularly  in  respect  to  the 
systems  of  radiating  drainage  seen  on  all  slopes  of  the  northern  moun- 
tains, is  suggestive  of  the  fact,  which  seems  borne  out  by  many  others, 
that  glacial  erosion  has  very  slightly  affected  the  sliape  of  these  greater 
reliefs  of  New  England.  The  accumulating  evidence  of  intense  glacial 
erosion  in  alpine  valleys,  whereby  hundreds  or  even  thousands  of  feet 
of  fresh  rock  have  been  quarried  away  by  master  glaciers  from  their 
rock  floors,  recalls  the  question,  raised  oftentimes  a  generation  ago,  as 
to  how  much  material  was  disturbed  by  the  great  Pleistocene  glaciej'S 
of  North  America.  The  answer  seems  again  to  be  unequivocal  tliat 
such  erosive  work  as  that  carried  on  during  Pleistocene  times  in  the 
Norwegian  fiords,  for  example,  was  not  paralleled  in  New  England. 
If  it  had  been,  we  should  expect  Mount  Ascutney,  once  entirely  over- 
ridden by  ice,  to  possess  a  somewhat  definite  stoss-and-lee  form  and 
to  have  suffered  a  serious  change  in  its  drainage.  The  radiating 
ravines  are  so  deep  and  contain  such  clear  evidences  of  glaciation  in 
their  bottoms  that  they  can  not  be  ascribed  to  post- Glacial  erosion. 
They  have  not  the  appearance  of  cirques,  and  hence  can  not  be 
ascribed  to  the  work  of  local  glaciers,  for  which,  indeed,  on  so  small 
a  mountain,  the  required  gathering  ground  is  lacking.  These  ravines 
and  water  courses  must  be  pre-Glacial.  This  being  the  case,  the  con- 
clusion lies  near  to  hand  that  the  Labrador  ice  sheet  did  not  approach 
in  erosive  activity  the  local  glaciers  of  Switzerland,  Norway,  Labrador, 
or  Alaska.  The  northwest,  north,  and  northeast  slopes  of  Ascutney 
would  have  borne  the  brunt  of  the  glacial  attack  and  perhaps  suf- 
fered a  7nore  vigorous  onslaught  than  the  lowlands  on  account  of  the 
projection  of  the  mountain  above  the  general  glaciated  floor,  but  these 
slopes  on  the  stoss  side  are  as  well  provided  with  the  usual  radiating 


DALY.]  CHAKACTEE   AND   HISTOKY   OF   THE   MOUNTAIN.  13 

stream  courses  as  those  on  the  south.  It  is  highly  improbable  that 
such  symmetry  would  persist  if  the  Ascutney  cone  had  beeu  seriously 
affected  in  volume  by  the  glacier.  The  same  patent  observation  can 
be  and  has  been  made  in  many  parts  of  northeastern  America  where 
the  apprppriate  reliefs  occur,  but  it  is  worthy  of  restatement  in  order 
to  point  out  once  again  the  mysterious  contrast  between  the  excavating 
power  of  i)resent  and  j)ast  valley  glaciers  and  of  the  incomparably 
greater  Pleistocene  ice  caps. 

The  mantle  of  glacial  drift  in  the  area  discussed  is  much  interrupted 
and,  in  general,  quite  thin,  so  that  outcrops  of  the  bed  rock  are 
numerous.  The  fine  terraces  of  the  Connecticut  and  of  Mill  Brook 
cover  some  of  the  "Calciferous  mica-schist"  (PI.  I,  B).  The  highest 
of  these  is  216  feet  (66  meters)  above  low-water  level  of  the  river  at 
Windsor.  It  was  used  by  J.  D.  Dana  as  important  evidence  of  the 
height  to  which  the  flooded  Connecticut  extended  its  banks  in  Cham- 
plain  times. '^^  The  gneisses  of  the  southwest  portion  of  the  area  are 
blanketed  over  with  the  alluvium  of  the  brook  at  Greenbush. 

SUMMARY. 

Mount  Ascutnej^  like  most  of  the  mountains  of  ISTew  England,  is  a 
residual  of  erosion,  a  monadnock  overlooking  a  dissected,  rolling 
plateau.  The  relief  as  a  whole  and  in  its  details  is  controlled  by  rock 
composition  in  a  specially  definite  manner.  Proofs  of  differential 
hardness  are  evident  in  the  present  topography,  intrusive  bodies 
contrasting  in  this  respect  with  one  another  and  with  tlie  adjacent 
schists.  Tlie  drainage  of  the  area  is  that  of  an  ancient  mountain 
system.  There  is  clear  adjustment  of  the  streamways  to  soft  struc- 
tures, giving  "longitudinal  subsequent"  streams  and  radially  diver- 
gent "  subconsequents. "  The  latter  occur  on  the  main  intrusive  rock 
body,  which  dominates  all  the  others  through  its  suiDerior  strength 
against  weathering  influences  and  through  its  relatively  greater  vol- 
ume. The  discussion  of  this  mountain,  which  is  but  one  of  a  numer- 
ous family  found  in  eastern  North  America,  emphasizes  once  more  the 
need  of  recognizing  deep-seated  intrusion  as  a  constructive  process  no 
less  important  for  certain  regions  than  the  faulting,  folding,  or  some 
other  initiating  deformation  of  the  earth's  surface  which  begins  a  new 
cycle  of  erosion.  The  history  of  the  Ascutney  topography,  including  its 
drainage,  begins  logically  and  chronologically  with  the  date  of  the 
intrusion  of  the  Main  syenite  stock.  The  existing  subconical  form 
and  the  radiating  stream  courses  of  the  mountain  may  be  said  to  be 
"subcousequent"  upon  that  constructional  process  of  intrusion. 

The  general  form  of  Ascutney  was  not  essentially  affected  by  the 
Pleistocene  giaciation.  A  veneer  of  pre-Glacial  weathered  rock  was 
removed  and  the  rounding  of  minor  points  accomplished  by  the  ice 
invasion,  but  the  pre-Glacial  Ascutney  had  practically  the  form  of 
the  present  mountain. 


a  Am.  Jour.  Sci.,  3d  series,  Vol.  XXIII,  1882,  p.  183. 


CHAPTER  II. 

GENERAL  DESCRIPTION   OF   THE  SCHISTS  IN  THE  AREA. 

The  fundamental  rocks  through  which  the  eruptions  took  place 
naturally  first  demand  attention.  The  following  account  of  them  will, 
however,  be  brief,  as  befits  the  main  purpose  of  this  paper.  They 
consist  of  two  conformable  members,  a  phyllitic  and  a  gneissic. 

PHXLLITIC  SJERIES. 

The  numerous  specimens  collected  from  the  phyllite  indicate  a 
tolerable  uniformity  in  the  lithological  character  of  that  rock  through- 
out its  whole  extent  in  the  neighborhood  of  Mount  Ascutney.  It 
is  composed  essentially  of  quartz,  sericite  (often  partially  replaced 
by  biotite),  argillaceous,  chloritic,  and  carbonaceous  material, 
accompanied  by  notable  amounts  of  iron  sulphides  and  titaniferous 
iron  ore  (PI.  II,  A).  Rare  crystals  of  orthoclase  and  of  a  triclinic 
feldspar,  equally  rare  grains  of  epidote,  and  perfect,  minute  crystals 
of  rutile  and  of  titanite  are  sporadically  developed.  The  quartz  forms 
interlocking  grains  between  the  sericite  fibers  and  layers  which  pro- 
duce the  marked  lamination  of  the  rock.  Straining  in  the  quartz  is 
at  times  notable  and  seems  to  be  correlated  with  microscopic  faulting 
in  the  rock  as  a  whole.  Along  these  incipient  fault  planes  a  further 
development  of  sericite  has  taken  place,  thus  giving  the  rock  the 
wavy  appearance  characteristic  of  strain-slip  cleavage  (PI.  II,  B). 
Good  examples  of  this  phenomenon  are  to  be  found  in  the  quarry 
beside  Mill  Pond  near  Windsor.  Lenses  and  laminae  of  milky  quartz 
are  very  abundant  and  have  sometimes  shared  in  the  crumbling  of 
the  phyllite,  though  generally  they  seem  to  have  been  formed  posterior 
to  its  folding. 

Very  often  through  the  series  the  argillaceous  material  is  nearly  or 
quite  absent  and  we  have  a  simi^le  quartz-sericite-schist.  An  excep- 
tionally fresh  specimen  of  this  phase,  collected  in  the  low  cliffs  just 
west  of  Ascutneyville  (spec.  24),  has  been  analyzed  (Table  I,  p.  15.) 
It  is  practically  a  quartzite,  which  bears,  in  addition  to  the  other 
essential  constituent,  sericite,  very  small  amounts  of  orthoclase,  an 
undetermined  plagioclase,  epidote,  ilmenite,  rutile,  titanite,  and  a  little 
pyrite,  with  probably  pyrrhotite. 
14 


DALY.J  PHYLLITIC    SERIES.  15 

Table  I. — Analysis  of  quartz-sericite-scMst  {specivien  24)  from  cliffs  ivest  of 

AsGutneyville. 

Per  cent. 

SiO^ -- -  90.91 

AlA 4.18 

FeA 0.22 

FeO 1.37 

MgO 0.37 

CaO 0.22 

Na^O 0.77 

K2O :---  0.58 

H2O  above  110^  C 0.74 

H2O  below  110°  C 0.06 

CO2 0.18 

TiO^ - 0.28 

ZrO/ 0.02 

PA ..  0.05 

CI , Trace. 

F Trace. 

'    FeSaandFe^Sg «0.11 

MnO .  -  Faint  trace. 

BaO . Trace. 

LiaO 1 strong  trace. 

0 '^O.IO 

100. 06 

Totals 0.056 

Sp.gr V 2.678 

The  phyllites  outcrop  on  the  edge  of  the  upper  terrace  of  the  Con- 
necticut River  with  a  strike  of  N.  5°  E.  and  an  average  dip  of  50"  E. 
(PI.  VII.) 

The  planes  of  foliation  here  and  all  across  the  series  to  the  gneissic 
area  are  regarded  as  very  closely  coincident  with  the  original  bedding. 
Many  beds  of  limestone  from  3  inches  to  2  feet  (8  to  60  centimeters) 
in  thickness  are  intercalated  in  the  phyllites  and  are  always,  so  far 
as  observed,  conformable  with  their  foliation  planes.  C.  H.  Hitchcock 
used  the  foliation  as  expressive  of  true  stratification  and  has  remarked : 
"I  believe  the  strata  on  both  sides  of  Ascutney  are  monoclinal  and 
dip  easterly."^  Farther  to  the  west,  in  the  monoclinal  structures  of 
the  Green  Mountains,  patient  and  successful  study  of  the  actual 
bedding  has  showed  a  close  correspondence  of  schistosity  and  true 
dip  in  the  many  folds  overturned  to  the  west.^  On  the  other  hand, 
there  is  no  doubt  that  in  other  localites  in  this  same  phyllitic  forma- 
tion there  is  ' '  striking  unconformity  between  the  planes  of  deposition 
and  the  fissility, "  ^ 

a  Note  by  Dr.  Hillebrand,  analyst:  On  boiling  with  dilute  HCl,  some  HoS  is  given  off,  followed 
by  a  strong  and  persistent  odor  of  volatilizing  sulpliui-,  showing  the  decomposition  of  a  sulphide 
with  the  formation  of  H^S  and  the  simultaneous  deposition  of  sulphur.  It  is  probable  that 
both  pyrite  and  pyrrhotite  are  present. 

b  Another  sample  gave  0.06  per  cent  carbon  and  0.42  per  cent  COo. 

•^Geology  of  New  Hampshire,  Concord,  1877,  Vol.  II,  p.  400. 

dSee  C.L.  Whittle,  The  genei-al  structure  of  the  main  axis  of  the  Green  Mountains:  Am.  Jour, 
Sci.,  3d  series,  Vol.  XLVn,  1894,  p.  347. 

eC.H.  Eichardson:  Proc.  Am.  Assoc.  Adv.  Sci.,  Boston  meeting,  1898,  p.  296. 


PLATE   II. 

A,  Unaltered  phyllite,  showing  normal  plane-parallel  strrictnre;  ordinary  light, 
X20.     (Seep.  14.) 

B,  Phyllite,  showing  bent  laminae  and  strain-slip   cleavage;    ordinary  light, 
X  20.     (See  p.  14.) 

16 


U.  S.  GEOLOGICAL  SURVEY 


BULLETIN   NO.  209,  FL.  II 


(^) 


(B) 


HE    MERIDEN    GRAVUfiE    CO. 


DALY.]  GNEISSIC    SEEIES.  17 

There  is  uo  better  summary  of  the  writer's  views,  obtained  from  but 
a  limited  stvidy  of  the  series  in  the  vicinity  of  Ascutney  ville,  than  that 
already  given  by  Edward  Hitchcock:  "We  have  noticed  no  cases 
where  the  stratification  and  schistose  structure  did  not  essentially 
coincide,  though  often  one  or  the  other  was  obscure,  ver}^  probably 
because  there  was  a  discordance  of  this  kind,  which  careful  study 
might  have  traced  out."^' 

Westward  across  the  strike  on  the  north  side  of  the  mountain  the 
dip  of  the  phyllite  is  seen  to  steepen  until  the  bedding  shows  an  incli- 
nation of  75°  or  more  to  the  east.  These  high  dips  occur  about  in  the 
meridian  passing  through  a  point  a  half  mile  east  of  Brownsville, 
where,  in  places,  the  dip  is  even  vertical.  The  strike  ranges  from 
N.  10°  E.  to  ]Sr.  23°  W.,  but  is  rarelj^  far  from  its  average  trend,  which 
is  due  north  and  south.  The  cross  section  on  the  south  side  of  the 
mountain  indicates  a  variation  in  the  strike  of  from  IST.  15°  E.  to  N. 
20°  W.,  with  the  average  again  practically  north  and  south.  The  dip 
averages  60°  to  the  east,  though  a  rapid  steepening  below  the  granite 
quarries  gives  angles  as  high  as  83°.  The  two  sections  of  the  phyllitic 
series  thus  correspond  to  a  dynamically  metamorphosed  integral  mass 
of  sediments,  deformed  so  as  to  present  the  appearance  of  a  great 
thickness  of  conformable  tilted  rocks  with  a  high  dip  to  the  east. 

GJ^EISSIC  SERIES. 

West  of  the  meridian  which  passes  through  the  diorite-syeuite 
contact  appears  a  group  of  medium-grained  to  coarse-grained  crystal- 
line schists  more  varied  in  composition  and  more  complex  in  structure 
than  the  phyllites.  There  is  no  distinct  plane  of  junction  between 
the  two  sei'ies.  Both  north  and  south  of  the  mountain,  going  west,  the 
phyllite  simply  assumes  a  more  and  more  feldspathic  character  until 
it  merges  into  a  conformable  and  typical  gneiss.  No  attempt  has 
been  made  to  unravel  this  complex,  even  so  far  as  that  is  possible. 
A  qualitative  treatment  only  has  been  deemed  sufficient  for  present 
purposes,  and  as  a  result  only  very  slight  differentiation  of  the  gneisses 
is  to  be  noted  on  the  map. 

The  most  abundant  member  of  the  gneissic  series  is  a  muscovite- 
biotite-epidote-gneiss  of  variable  texture.  It  is  often  richly  charged 
with  scapolite.  Likewise  abundant  are  biotite-muscovite-gneiss, 
biotite-gneiss,  muscovite-gneiss,  epidote-gneiss,  or  mica-schists,  all  of 
which  seem  to  be  transitional  into  the  main  gneissic  type.  Very  often 
the  feldspars  are  large  and  the  structure  is  that  of  a  true  augengneiss. 
All  these  types  may  be  garnetiferous.  With  them  are  associated  thin 
bands  of  beautifully  crystallized  hornblende-biotite-quartz-schists  and 
epidotic  hornblende-schists.  The  finest  types  of  these  hornblende- 
schists  were  found  in  a  number  of  massive  ledges  on  the  west  side  of 

1  Geology  of  Vermont,  Claremont,  1861,  Vol.  I,  p.  476. 

Bull.  209—03 2 


18  GEOLOGY    OF    ASCUTNEY   MOUNTAIN,   VERMONT.         [bull.  209. 

the  road  running  through  Greenbush,  and  thus  outside  the  limits  of 
the  area  mapped,  but  excellent  sill-like  occurrences  may  be  studied 
just  below  the  Crystal  Cascade.  Thick  pods  of  coarsely  crystalline 
limestone  and  of  marble,  generally  charged  with  nests  of  radiating 
tremolite  (the  "wood-rock"  of  the  quarryman),  are  included  in  the 
gneissic  area,  but  again  outside  the  immediate  region  under  considera- 
tion. The  nearest  of  these  lenses  of  limestone  has  been  rather  exten- 
sivel}^  quarried  for  the  manufacture  of  quicklime  at  Amsden,  about  2 
miles  southwest  of  Little  Ascutney .  Sheets  of  now  greatly  weathered 
diabase  are  not  uncommon  in  the  gneisses,  and  the  apophyses  from 
the  diorites  and  syenites  often  assume  the  same  form. 

An  intrusive  sheet  of  composition  and  origin  quite  different  from 
that  of  any  other  rock  yet  studied  in  the  area  was  found  exposed  for  a 
length  of  about  500  yards  (458  meters)  with  a  strike  of  N.  15°  W.  It  is 
terminated  at  the  northern  end  by  the  younger  rock  mass  of  the  Main 
syenite  stock  at  a  point  about  one-half  mile  from  Crystal  Cascade. 
The  southern  end  of  the  sheet  is  concealed  by  a  drift.  This  sheet 
varies  from  60  to  100  feet  in  thickness  and  dips,  conformably  with  the 
quartz-epidote-schist,  55°  to  the  east.  The  inclusion  within  its  mass 
of  horses  of  the  schist  and  the  appearance  of  apophyses  from  it  within 
the  latter  clearly  prove  its  intrusive  nature  (see  PL  VII). 

A  gneissic  structure  is  generally  visible  on  weathered  surfaces, 
though  it  is  sometimes  quite  absent.  The  sheet  rock  has  evidently 
been  squeezed  with  the  schist,  and  both  have  been  broken  by  faults  of 
small  throw.  The  light-gray  hand  specimen  itself  exhibits  granula- 
tion, and  the  extensive  alteration  of  the  old  eruptive  is  indicated  by  the 
presence  of  many  irregular  grayish  to  silvery- white  blotches  of  mus- 
covite  (specimen  175).  Dark-colored  minerals  are  not  visible  macro- 
scopically.  Microscopic  examination  shows  that  the  crushing  has 
been  j)rofound.  Abundant  granulated  quartz  and  orthoclase,  greatly 
bent  lamellse  of  plagioclase  and  microcline  ('?),  and  the  abundant  mus- 
c-ovite  present  especially  characteristic  proofs.  Besides  epidote,  which 
appears  to  be  a  metamorphic  derivative  from  the  plagioclase,  rare 
zircon  crystals  and  a  few  grains  of  an  iron  ore  complete  the  list  of 
constituents.  A  muscovite-gneiss  at  the  present  time,  this  rock  was 
doubtless  of  the  nature  af  an  aplitic  sill  before  the  period  of  dynamic 
metamorphism.  It  is  of  interest  in  representing  something  like  the 
condition  to  which  the  latter  eruptives  would  have  been  reduced  if, 
since  intrusion  and  consolidation,  they  had  been  affected  by  mountain 
building  on  the  scale  indicated  by  the  present  attitude  of  the  schists. 

It  has  alreadj^  been  noted  that  on  both  sides  of  the  mountain  the 
phyllite  assumes  from  east  to  west  a  more  and  more  gneissic  character 
in  a  fairly  broad  north-south  zone.  The  strike  and  dip  in  this  transi- 
tion zone  are  similar  to  those  of  the  phyllites  proper,  and  they  are 
retained  in  the  gneisses  on  the  south  of  the  dioritic  stock.  On  the 
north  side,  however,  while  conformable  with  the  phyllite  near  the 


DALY.]         AGE    OF    THE    SCHISTS    AND    THE    INTRUSIVE    ROCKS.  19 

transition  zone,  the  gneisses  vary  in  strike  from  north  and  south  to 
east  and  west.  The  average  strike  is  about  northwest  to  southeast. 
As  the  section  is  followed  westward  the  gneisses  are  seen  to  be  greatly 
contorted  and  to  writhe  about  in  the  most  irregular  way,  the  angle  of 
dip  changing  considerably  with  the  strike.  These  structural  changes 
are  introduced  so  gradually  in  and  west  of  the  transition  zone  that 
they  do  not  preclude  the  idea  that  the  gneiss  underlies  the  phyllites 
conformably.  Such  is  believed  to  be  the  relation  between  the  two 
series. 

In  this  instance  the  chronological  treatment  of  the  rocks  of  the 
area  has  been  deviated  from.  This  has  been  done  because  the  phyl- 
lites have  greater  stratigraphic  simplicity,  and  dynamic  metamorphism 
has  affected  them  to  a  much  smaller  degree  than  the  older  crystalline 
schists.  At  the  same  time,  the  amount  of  light  thrown  on  the  origin 
of  the  gneisses  by  the  brief  and  limited  study  of  the  phyllites  is  not 
that  which  would  accrue  from  an  accurate  and  detailed  mapping  of 
the  schists  far  beyond  the  limits  of  the  area  mapped.  The  intimate 
association  of  the  two  series  and  the  occurrence  of  the  limestone  pods 
in  the  gneiss  render  it  highly  probable  that  the  gneiss  is  for  the  most 
part  composed  of  material  that  was  originally  sedimentary.  Beyond 
this  general  statement  the  facts  obtained  in  the  Ascutney  area  will 
not  permit  us  to  go,  nor  for  the  immediate  purpose  of  this  paper  is  it 
necessary  to  inquire  further  into  the  details  of  the  history  of  the 
metamorphism. 

GEOLOGIC  AGE  OF  THE  SCHISTS  AND  OF  THE    INTRUSIVE 

ROCKS. 

Outside  regions  must  be  turned  to  for  a  solution  of  the  difficult 
problem  as  to  the  age  of  the  schists  and,  inferentially,  as  to  the  maxi- 
mum age  that  may  be  assigned  to  the  eruptives.  The  Vermont  Sur- 
vey Report  of  1861  includes  the  phyllitic  series  in  the  "  Calciferous 
mica  schist,"  and  states  that  this  formation  is  overlain  by  clay  slate 
which  a  "strong  presumption "  would  place  in  the  Devonian;  and 
hence  that  the  schist  is  at  least  Devonian,  and  may  be  older. «  Speak- 
ing of  the  underlying  gneiss,  Edward  Hitchcock  wrote,  ' '  We  have 
already  made  it  probable  that  the  Calciferous  mica  schist  has  been 
converted  into  gneiss  from  Ascutney  southward.  If  so,  whatever  the 
age  of  the  schist  may  be,  that  of  the  gneiss  is  the  same."^  Hitchcock 
left  the  question  of  age  open,  though  he  seems  to  have  given  weight 
to  T.  S.  Hunt's  conclusion,  based  on  studies  in  the  northern  extension 
of  these  schists  into  Canada,  that  they  are  of  Niagara  or,  at  any  rate, 
Upper  Silurian  age.'^  Emerson  has  proved  that  the  Bernardston  series 
of  Vermont  and  Massachusetts  is  of  Hamilton  and  Chemung  age.'^ 
That  series  overlies  the  Calciferous  mica  schist,  so  that  the  latter  is 
Lower  Devonian  or  older. 

« Vol.  I,  p.  485.  c  Am.  Jour.  Sci.,  2d  series,  Vol.  XVIII,  1854,  p.  198. 

b  Geology  of  Vermont,  1861,  Vol.  I,  p.  470.       a  Idem,  Vol.  XL,  1890,  p.  263. 


20  GEOLOGY    OF    ASCUTNEY^MOUNTAIlSr,   VEEMONT.         [bull.209. 

Recently  the  detailed  field  observations  of  C.  H.  Richardson  have 
afforded  more  definite  information.  He  divides  the  Calciferous  mica 
schist  into  a  calcareous  member,  the  Washington  limestone,  and  a  non- 
calcareons  member,  the  Bradford  schist.  The  latter  includes  the 
phyllitic  series  of  the  Ascutney  region.  Richardson  correlates  the 
Bradford  schist  with  the  Goshen  schist  of  Emerson  in  Massachusetts. 
The  Bradford  schist  is  "flanked  on  the  east  by  a  band  of  clay  slate 
and  on  the  west  by  the  Washington  limestone,  which  in  turn  is 
flanked  on  the  west  by  a  band  of  clay  slate.  The  two  bands  of  slate, 
the  Bradford  schist,  and  the  Washingtoii  limestone  lie  unconf ormably 
both  on  the  east  and  west  on  a  synclinal  trough  of  the  hydromica- 
schist,  which  is  Hurouian."  His  discovery  of  fossils  in  the  clay  slate 
has  enabled  Richardson  to  prove  it  to  belong  to  the  Lower  Silurian. 
"The  Bradford  schist  and  Washington  limestone,  which  have  oscil- 
lated from  the  '  Primitive '  of  Zadock  Thompson  to  the  Niagara  of 
Professor  Dana,  are  Lower  Silurian,  and,  more  definitely.  Lower 
Trenton. "« 

The  schists  had  been  flexed  into  essentially  their  present  attitude 
before  any  considerable  eruption  of  igneous  rock  took  place  in  this  area. 
The  sheets  of  amphibolite  and  sills  of  aplitic  material  noted  above 
have  been  changed  both  in  composition  and  structure  by  the  dynamic 
action  of  the  tilting  process.  They  are,  however,  of  minor  impor- 
tance, and  do  not  weaken  the  conclusion  that  the  mountain-buiJding 
forces  had  practically  ceased  acting  before  the  eruptive  masses  of 
Ascutney  had  appeared  in  the  main  conduit.  Occasionally  the  feld- 
spars and  biotite  crystals  of  the  rock  in  the  oldest  (gabbro-diorite) 
stock  show  considerable  straining  and  bending,  but  such  effects  are 
far  inferior  in  degree  to  those  which  would  result  if  the  diorite  had 
undergone  the  enormous  pressure  necessitated  in  the  folding  of  the 
sediments.  The  still  younger  intrusives  show  even  less  evidence  of 
squeezing  or  dislocation  since  they  were  consolidated. 

The  principal  intrusions  are,  accordingly,  post-Trenton  in  age,  and 
probably,  if  we  may  judge  from  the  analogy  of  other  granitic  intru- 
sions in  the  Appalachian  system,  post-Carboniferous  and  pre-Creta- 
ceous.     Nearer  than  that  they  can  not  as  yet  be  more  definitely  dated, 

SUMMARY. 

The  irruptives  of  Mount  Ascutney  cut  a  series  of  tilted  schists 
assigned  to  horizons  equivalent  with  that  of  the  Bradford  schist  or 
older;  i.  e.,  they  are  regarded  as  Trenton  or  pre-Trenton  in  age.  The 
overlying  phyllites — the  "Calciferous  mica-schist"  of  the  geological 
survey  of  Vermont — belong  to  the  Bradford  schist  proper.  While 
highly  metamorphic  and  greatly  deformed,  the  phyllites,  with  the 
interbedded  quartzitic  and  thin  limestone  bands,  show  an  api^arent 

'^ Proc.  Am.  Assoc.  Adv.  Sci.,  Vol.  XL VII,  1898,  p.  296. 


DALY.  J 


AGE    OF   THE    INTRUSIONS.  2l 


parallelism  between  scliistosity  planes  and  stratification  planes. 
Beneath  this  series  is  a  conformable  group  of  schistose  rocks  con- 
sisting of  common  mica-gneisses,  epidote-gneisses,  amphibolites,  and 
crystalline  limestone. 

The  intrusions  are  of  later  date  than  the  last  great  period  of  rock 
folding  which  has  affected  the  Ascutney  region,  and  the  balance  of 
probability  makes  them  post-Carboniferous  and  pre-Cretaceous  in 
age. 


CHAPTER  III. 

CONTACT  METAMORPHISM. 
THE  METAMORPHIC  AUREOLE. 

The  metamorpliic  aureole  developed  about  the  stocks  is  worthy  of 
detailed  consideration.  As  one  approaches  the  contact  from  any  side 
of  the  mountain  he  speedily  notes  plentiful  evidences  of  the  squeezing, 
crumpling,  and  fracturing  of  the  schists,  which  thus  have  a  more  com- 
plex structure  than  they  have  at  some  distance  away  from  the  massive 
rocks.  Especially  good  examples  of  this  may  be  seen  on  the  cleared 
spurs  on  the  southeast  side  of  Ascutney  Mountain  proper.  There,  as 
elsewhere,  further  jjroof  of  the  energetic  nature  of  the  intrusions  is  to 
be  found  in  the  numerous  apophyses  sent  into  the  bedded  series  and 
in  the  disruption  of  blocks,  great  and  small,  from  the  latter,  often 
forming  a  "permeation  area"  in  the  eruptive  masses;  but  the  pur- 
pose of  the  present  chapter  relates  not  so  much  to  the  dynamics  of 
the  metamorpliic  action  as  to  the  mineralogical  changes  which  have 
taken  place  in  the  schists. 

The  heat  and  the  mineralizers  accompanying  the  intrusions  have  pro- 
duced alterations  which  are  most  important  in  the  phyllites  and  asso- 
ciated limestones.  On  account  of  the  well-known  mineralogical 
stability  of  the  gneisses  and  of  the  quartzitic  bands  the  metamorphic 
effects  upon  these  rocks  have  been  mechanical  rather  than  chemical 
or  mineralogical. 

The  breadth  of  the  aureole  is  not  great  in  any  part.  Indisputable 
contact  metamorphism  has  not  anywhere  been  recognized  at  much 
over  GOO  feet  (183  meters)  from  the  contact,  and  may  often  be  dis- 
tinctly seen  no  more  than  300  feet  (91  meters)  away  from  the  same 
line.  •  In  the  phyllitic  series  the  metamorphic  belt  averages  about  500 
feet  (154  meters)  in  width,  and  that  irrespective  of  the  attitude  of  the 
schists  and  irrespective  of  the  stock  nearest  to  which  the  measure- 
ment is  made.  The  Main  syenite  stock  has  controlled  the  metamor- 
phic action,  although  the  Basic  stock  seems  to  have  slightly  intensified 
the  action  at  the  triple  contacts  of  syenite,  phyllite,  and  diorite.  It 
is  essentially  correct  to  speak  of  the  metamorphism  of  the  phyllites 
as  resident  in  one  aureole  produced  by  the  intrusion  of  the  Main 
stock. 
22 


daly]  contact  metamoephism.  28 

cha:n^ges  in^  the  eimestoxes. 

The  interbedcled  calcareous  layers  of  the  phjdlitic  series  are  spe- 
cially sensitive  to  this  contact  transformation.  Two  different  rock 
types  result  from  the  alteration  of  the  fine-grained  bluish-gray  sili- 
ceous limestone,  which  is  composed  simi3ly  of  ealcite,  quartz,  and 
carbonaceous  matter  (spec.  114). 

The  first  of  these  phases  of  recrystallization  has  resulted  from  an 
abundant  replacement  of  the  ealcite  by  epidote,  the  other  constituents 
remaining  unchanged.  This  variety  was  found  at  several  points  in 
the  aureole.  Of  the  occurrences  known,  that  farthest  removed  from 
the  syenite  lies  about  600  feet  (183  meters)  from  the  contact  at  the 
base  of  the  prominent  2,350-foot  spur  bearing  east  by  north  of  the 
summit.  A  very  similar  phase  was  discovered  only  3  feet  (0.9  meter) 
from  the  contact  at  the  Crystal  Cascade.  Here  scapolite  is  also  devel- 
oped and  rare  grains  of  titanite  and  brownish-green  hornblende  are 
accessory  (spec.  100). 

The  second  phase  is  illustrated  in  a  specimen  from  a  field  just  west 
of  the  old  road  leading  up  to  the  quarries  in  the  granite.  The  ledge 
from  which  it  was  taken  is  450  feet  (137  meters)  from  the  contact  with 
that  intrusive.  Attention  was  first  called  to  it  by  the  very  noticeable 
roughness  of  the  weathered  surface,  which  betrayed  the  jpresence  of 
some  mineral  in  roundish  masses  more  resistant  to  the  weather  than 
the  other  constituents  (spec.  5).  The  fresh  specimen  is  light  graj', 
mottled  with  these  subcircular,  rather  darker-colored,  oily-looking 
areas  about  6  millimeters  in  diameter  and  at  an  average  distance  of 
1  to  2  centimeters  apart.  Under  the  microscope  the  areas  resolve 
themselves  into  irregular  aggregations  of  the  colorless  to  yellowish 
lime-garnet  (grossularite),  inclosing  large  amounts  of  ealcite.  The 
occurrence  is  thus  similar  to  that  described  by  Harker  and  Marr  in 
the  metamorphosed  limestone  of  the  Shap  Fell  region  in  England" 
and  to  many  others  of  different  localities. 

The  garnet  never  shows  crystal  form,  nor,  except  in  rare  cases,  any 
distinct  cleavage.  The  usual  optical  anomalies  and  zonal  extinction 
are  present.  The  anisotropic  propertj^  is  very  unevenly  distributed 
throughout  the  areas.  It  may  be  completely  absent  in  one  grain  and 
give  a  polarization  tint  like  that  of  zoisite  in  a  contiguous  individual. 
The  double  refraction  shows  that  in  SQpae  instances  the  areas  are 
occupied  by  single  poikilitic  crystals  of  the  grossularite  as  shown  by 
the  uniform  extinction  on  revolving  the  section  between  crossed  nicols. 
The  same  test  would  seem  to  indicate  the  comj)osite  character  of  other 
masses  which  are  aggregates  of  small  individuals.  The  only  other 
change  from  the  normal  limestone  is  the  complete  disapj)earance  of 
carbonaceous  matter.  Quartz  remains  in  relatively  high  proportion 
and  occurs  with  ealcite  as  inclusions  in  the  garnet. 

a  Quart.  Jour.  Geol.  Soc,  Vol.  XL VII,  1891,  p.  311. 


24  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [BnLL.209. 

METAMORPHISM   OF   THE   PHXELITES. 

The  average  normal  pliyllite  is,  as  we  have  seen,  considerably  more 
argillaceous  than  the  quartzitic  schist  analyzed  (see  Table  I,  p.  15). 
The  minerals  characterizing  the  rocks  resulting  from  the  alteration 
are,  as  a  rule,  those  which  might  be  expected  from  mere  recrystalliza- 
tion  of  the  original  constituents,  viz,  quartz,  sericite,  chlorite,  clayey 
matter,  iron  ores,  and  sulphides. 

There  is  no  definite  succession  of  zones  of  metamorphic  action, 
either  of  color,  structure,  or  mineral  aggregation,  as  in  the  classic 
region  of  Barr  Andlau.  The  macroscopic  changes  are  simple  and 
uniform  in  all  parts  of  the  aureole.  Quartz  veins  and  eyes  become 
morf^  nujnerous  as  the  contact  is  approached;  the  lamii  .tion  of  the 
schist  becomes,  at  the  same  time,  more  and  more  lost,  and  the  rock 
takes  on  an  increasingly  compact  and  indurated  look.  Yet  even  at 
the  contact  itself  the  presence  of  quartzose  laminae  in  the  original 
phyllite  often  entails  a  partial  preservation  of  the  schistose  structure. 
Occasionally  obscure  spotted  and  knotted  areas  are  found,  but  they 
are  not  conspicuous  nor  are  they  arranged  in  any  fixed  order. 

The  general  mineralogical  changes  may  be  summarized  as  compris- 
ing a  progressive  disapi3earance  of  sericite,  quartz,  and  argillaceous 
substance  and  a  corresponding  development  of  biotite,  red  garnet, 
cordierite,  pleonaste,  corundum,  and  sillimanite.  These  new  minerals 
naturally  occur  most  abundantly  and  in  larger  crystals  near  the  con- 
tact than  farther  out  in  the  aureole.  In  tracing  these  changes,  the 
attempt  was  made  to  collect  specimens  along  the  strike,  thus  inviting, 
though,  on  account  of  the  variability  and  disturbed  character  of  the 
schist  and  because  of  the  lack  of  sufiicient  outcrops,  not  entirely 
securing,  the  maximum  of  certainty  as  to  just  how  great  has  been  the 
influence  of  this  local  metamorphism  on  lithological  units.  Three 
fairly  representative  sections  of  the  contact  zone  were  made  in  this 
way ;  perhaps  no  better  means  of  describing  the  phenomena  of  the 
zone  can  be  adoj)ted  than  to  consider  each  of  the  sections  somewhat 
in  detail. 

SERIES  A  OF  SPECIMENS   FROM   THE  METAMORPHIC  AUREOLE. 

The  first  of  these  sections  in  the  aureole  is  noted  on  the  geological 
map  as  occurring  at  the  syenite  contact  on  the  north  side  of  the  moun- 
tain; the  set  of  specimens  collected  there  may  be  called  "Series  A." 
For  ease  of  reference  each  of  the  following  ]3aragraphs  relating  to  the 
description  of  the  specimens  is  preceded  by  a  number  indicating  the 
distance  from  the  contact  of  the  specimen  to  which  that  paragraph 
refers. 

500  feet  {15 Jf 'meters). — Five  hundred  feet  from  the  contact  the  phyl- 
lite shows  some  crumpling  and  otlier  evidences  of  disturbances;  so 
far  as  known,  however,  no  new  mineral  has  been  developed  at  that 
distance. 


DALY.]  SPECIMENS    FEOM    THE    METAMORPHIC    AUREOLE.  25 

JfiO  feet  {19,'B  meters). — One  hundred  feet  nearer,  the  sericite  is 
largely  replaced  by  an  indeterminable  chloritic  snbstance,  but  it  is 
probable  that  this  phase  also  is  original. 

SOO  feet  {91  meters). — Somewhere  within  the  next  100  feet  measured 
toward  the  contact  there  is  a  comijaratively  abrupt  appearance  of 
true  contact  minerals,  coupled  with  a  decided  loss  of  the  original 
fissility  of  the  rock  (spec.  129).  At  300  feet  from  the  syenite,  there 
is  a  partial  replacement  of  the  argillaceous  and  chloritic  material 
and  of  quartz  by  cordierite,  while  the  whole  rock  is  filled  with  a 
swarm  of  extremely  minute,  light-green,  isotro]3ic  grains  with  high, 
single  refraction.  An  occasional  grain  of  epidote  lies  with  the 
iron  sulphides  (pyrite  and  probably  pyrrhotite)  in  the  planes  of 
schistosity,  which  are  still  to  be  seen,  both  macroscopically  and  in  tlie 
thin  section. 

250  feet  {77  meters). — Fifty  feet  nearer,  the  metamorphism  has 
affected  the  whole  rock.  Its  original  dark-gray  color  has  now  a  bluish 
cast  (spec.  128) .  A  high  degree  of  induration  with  a  corresponding  loss 
of  lamination  and  an  increase  in  the  specific  gravity  are  characteristic. 
A  peculiar  feature  of  this  hand  specimen  is  the  presence  of  numer- 
ous roundish  and  isolated  areas  of  what  can  be  discerned,  even 
macroscopically,  to  be  granitic  aggregates  of  quartz,  feldspar,  and 
other  minerals  embedded  in  the  general  rock  matrix.  They  bear  a 
relation  to  the  transformed  schist  analogous  to  that  of  a  miarole  to 
its  igneous  host,  and,  for  lack  of  a  better  term,  they  vaay  be  called 
"pseudo-miaroles."  Microscopic  examination  shows  that  the  basis 
of  the  rock  is  now  cordierite  occurring  in  interlocking  individuals 
2  to  3  millimeters  in  diameter.  It  is  always  poikilitic  either  from 
the  mutual  intergrowth  of  several  crystals  of  its  own  substance 
or  from  the  swarms  of  mineral  inclusions  of  different  sorts  (the 
"sieve-structure"  of  Salomon).  Those  inclusions  are  the  same  as 
the  other  constituents  of  the  hornfels,  viz,  numerous  small  shreds 
of  intensely  pleochroic  brown  biotite,  abundant,  irregular  grains  of 
deep-green  sj)inel,  pyrite,  pyrrhotite,  ilmenite,  tourmaline  of  brown 
and  yellow  tones,  quartz,  and  minute  black,  probably  carbonaceous 
particles. 

These  various  inclusions  cloud  the  whole  thin  section  except  in  the 
more  quartzose  laminse,  which  are  doubtless  residual  from  the  origi- 
nal rock,  and  in  the  pseudo-miaroles.  They  are  also  lacking  in  the 
numerous  stringers  of  quartz  which  traverse  the  schist.  Probably 
the  recrystallization  of  the  schist  was  complete  before  the  quartz  was 
laid  down  in  the  veinules  and  the  pseudo-miaroles  were  filled  with 
their  granitic  contents.  The  latter  consists  of  the  minerals  charac- 
teristic of  the  adjacent  syenite — microperthite,  cryptoperthite,  quartz, 
brown  alkaline  hornblende,  and  rare  zircons.  With  them  is  often 
associated  a  pale-reddish  garnet  averaging  about  1  millimeter  in 
diameter.     The  quartz  is  so  abundant  as  to  give  the  hypidiomorphic- 


26  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,  VERMONT.         [bull.209. 

granular  aggregate  the  composition  of  a  true  granite.  These  pseudo- 
miaroles  when  round  in  outline  measure  from  3  to  5  millimeters  in 
diameter;  when,  less  often,  they  are  elongated  in  the  section  they 
measure  from  5  to  10  millimeters  in  length.  They  are  not  connected 
with  one  another  or  with  distinct  apophysal  veins  from  the  syenite, 
but  are  completely  surrounded  by  the  cordieritic  matrix.  It  looks  as 
if  there  had  been  a  shrinkage  of  volume  in  the  schist  during  its 
recrystallization  and  the  resulting  cavities  were  subse4uently  filled 
with  the  granitic  substance  by  a  pneumatolytic  process.  We  have, 
whatever  be  the  explanation,  a  striking  case  of  feldspathization  of 
schist  by  an  intrusive  granitic  rock  wherein  the  channels  of  approach 
of  the  feldspathic  material  were  of  submicroscopic  dimensions.  The 
deep-green  spinel  is  pleonaste,  which  is  now  of  increasing  importance 
as  we  go  toward  the  eruptive  rock.  It  was  this  mineral  that  war 
seen  at  the  300-foot  distance,  where  the  very  small  grains  were  inde- 
terminable. Similar  fine  material  is  present  here,  but  it  grades  up 
into  larger  individuals  which  are  undoubtedly  pleonaste.  Here,  too, 
the  pleonaste  has  an  interesting  localized  distribution,  being  grouped 
in  roundish  clusters  composed  of  many  crystals.  In  one  case  the 
whole  of  one  well-marked  cluster  about  0.3  of  a  millimeter  in  diam- 
eter is  included  in  a  single  crystal  of  cordierite.  The  spinel  is 
usually  without  crystal  form  and  occurs  as  drop-like  bodies.  It  is 
worthy  of  note  that  not  only  in  this  case,  but  in  all  parts  of  the 
aureole,  pleonaste  and  the  metamorphic  biotite  are  in  reciprocal 
relation  to  each  other ;  where  one  is  abundant  the  other,  relatively,  is 
rare.  This  fact  correlates  with  the  observation  of  Lacroix  that  spinel 
is  a  common  product  of  the  alteration  of  mica  in  inclusions  caught 
up  in  lavas,"  and  with  our  own  observation  that,  close  to  the  contact, 
where  we  should  expect  the  more  stable  products  of  metamorphism, 
the  biotite  is  often  completely  replaced  by  pleonaste. 

150  feet  {J/-6  meters). — One  hundred  feet  nearer  the  contact  the  schist 
is  macroscopically  similar  to  the  rock  found  at  250  feet  in  its  compact- 
ness, lack  of  pronounced  schistosity,  and  bluish-gray  color,  but  lacks 
the  pseudo-miaroles  and  is  more  strongly  charged  with  the  red  garnet 
(spec.  127).  Mineralogically  the  most  important  difference  is  found  in 
the  entrance  of  corundum  as  a  new  metamorphic  constituent.  This 
occurs  as  irregular  colorless  grains,  often  grouped  in  clusters  about 
ilmenite  in  the  form  of  a  mantle.  This  new  mineral  is  in  small  amount 
and  its  description  may  be  deferred  for  better  occurrences  at  other 
localities.  Cordierite  again  composes  most  of  the  schist.  It  has  here, 
too,  the  pleochroic  lialos  found  about  inclusions,  as  well  as  other  usual 
features.  Pleonaste,  with  a  clustering  habit,  is  comparatively  abun- 
dant, and  biotite  is  rare.  The  garnets  appear  in  large  individuals, 
showing  characteristic  cleavage  and  inclusions,  and  also  in  the  form  of 
well-crystallized  minute  dodecahedrons  without  inclusions  or  cleavage. 

«  Les  enclaves  des  roches  volcaiiiqiies,  Macon,  1893,  p.  599. 


DAiA-.]  SPECIMENS    FEOM   THE    METAMOEPHIC    AUREOLE.  27 

Manj^  of  the  larger  crystals  form  the  centers  of  eyes  and  are  then 
wrapped  about  by  mica  plates.  Generally,  however,  the  garnet  is  sur- 
rounded by  a  clear  zone  of  quartz  and  cordierite,  unaccompanied  by 
iron  compounds,  which  have  either  been  used  to  build  up  the  garnet 
or  are  included  in  zones  within  that  mineral.  Though  the  pseudo- 
miaroles  are  absent,  there  is  evidence  of  some  feldspathization  of  the 
schist.  Occasional  crystals  of  microperthite  may  be  discerned  in  the 
thin  section.  They  are  products  of  late  crystallization,  as  they  are 
intersertally  related  to  the  cordierite ;  like  the  feldspars  of  the  pseudo- 
miaroles,  they  are  free  from  inclusions  of  pleonaste,  etc. 

100  feet  {31  meters.) — A  specimen  (No.  126)  taken  from  a  ledge  100 
feet  from  the  contact,  seems  in  several  respects  to  show  a  local  excep- 
tion to  the  general  effect  of  metamorphism  on  the  phyllites.  Biotite  is 
once  more  developed  in  profusion,  while  pleonasteis  quite  subordinate. 
Corundum  is  absent.  Cordierite  is  again  the  principal  constituent 
and  acts  as  host  to  the  pleonaste  clusters.  The  iron  sulphides  are 
conspicuous  in  the  hand  specimen.     Dr.  Hillebrand  remarks : 

On  boiling  the  powdered  rock  with  dilute  hydrochloric  acid  considerable  H2S 
is  evolved  and  sulphur  is  set  free,  as  can  be  plainly  perceived  by  the  strong  smell  of 
volatilizing  sulphur  accompanying  that  of  HjS.  The  decomposition  begins  at  a 
moderate  heat.  After  a  time  the  evolution  of  the  HjS  ceases  and  the  smell  of 
sulphur  is  no  longer  noticeable,  and  there  is  then  found  only  one-third  of  the  total 
sulphur  left  in  the  residue,  presumably  wholly  as  pyrite.  The  rest  of  the  sulphide 
is  magnetic  and,  dissolving  readily  in  HCl  with  the  evolution  of  HjS,  may  be 
considered  as  quite  certainly  pyrrhotite. 

The  feldspar  is  much  increased  in  amount,  is  notably  mieroperthitic, 
and  occurs  in  the  same  relations  as  in  the  last  specimen  described. 
For  the  first  time,  the  feldspar  shows  the  Carlsbad  twins  characteristic 
of  the  microperthite  of  the  syenites.  A  chemical  analysis  has  been 
made  of  this  phase  (Table  II,  col.  1).  As  a  whole  it  corresponds  to 
the  analysis  of  a  phyllite  rich  in  argillaceous  material.  The  soda  is 
to  be  ascribed  mainly  to  the  feldspar,  and  is  thus  believed  to  have 
been  introduced  by  hydrothermal  action.  On  the  supposition  that  all 
tlie  sulphide  exists  as  pyrite,  the  proportions  of  the  iron  compounds 
would  be — 

Per  cent. 

FejOg 0.03 

FeO 6.41 

FeS^ 0.58 

If  two-thirds  of  the  sulphide  is  pyrrhotite,  these  compounds  should 
be  recalculated  to  the  following  proportions : 

Per  cent. 

Fe,03 0.30 

FeO -----. 6.00 

FeS^ • 0.19 

Fe7S8 0.53 


28  GEOLOGY    OF    ASCUTNEY    MOUNTAm,   VERMONT.         [bull. 209. 

The  latter  proportions  are,  for  tlie  reason  already  noted,  believed 
to  be  more  nearly  correct  and  are  accordingly  entered  in  the  total 
analysis.  The  carbon  percentage  is  extremely  variable,  correspond- 
ing to  the  irregular  distribution  of  the  coaly  matter  in  the  schist.  One 
independent  determination  gave  0.03  per  cent  carbon  instead  of  0.40 
per  cent,  as  found  for  the  rock  fragment  analyzed  completely.  Two 
analyses  of  typical  cordierite-mica-hornfels  from  southern  Carinthia 
(Table  II,  cols.  3  and  4),  show  a  rather  strong  similarit}^  to  that  of  the 
Ascutney  rock. 

50  feet  {15.5  meters). — Fifty  feet  from  the  contact  a  specimen  (No. 
125)  was  collected  that  showed  a  reversion  to  the  normal  sequence  of 
the  mineral  occurrences  as  the  contact  is  approached.  The  pleonaste 
once  more  largely  supijlants  the  biotite  and  is,  in  fact,  more  abundant 
than  ever.  It  clouds  the  whole  of  the  thin  section,  though  there  is  a 
tendency  toward  a  grouping  along  planes  apparently  representing  the 
original  schistosity.  Corundum  and  tourmaline,  like  the  biotite, 
occur  but  sparingly.  The  schist  is  strongly  impregnated  with  small 
quartz  veins  and  with  lenses  1  to  2  millimeters  in  thickness,  composed 
of  quartz,  microperthite,  and  brown  biotite.  The  evidence  is  not  so 
clear  as  in  the  case  of  the  pseudo-miaroles  that  these  granite  lenses 
are  not  actually  connected  with  one  another  and  with  the  stock  rock, 
but  it  is  highly  probable  that  both  types  of  the  granitic  aggregates 
are  to  be  referred  to  the  same  pneumatolytic  origin. 

25  feet  {8  meters). — Halfwaj^  to  the  contact  the  rock  is  still  the 
massive  dark  bluish-gray  heavy  hornfels,  hardly  to  be  distinguished 
in  the  hand  specimen  from  the  altered  schist  collected  from  the  200 
or  more  feet  of  section  over  which  we  have  just  iDassed  (spec.  124). 
A  notable  difference  from  the  hornfels  at  50  feet  ■  consists  in  the 
abundance  of  corund«um,  which  is  even  more  plentiful  than  the  pleo- 
naste. Cordierite  is  here  again  the  chief  constituent.  While  it  pre- 
sents the  usual  poikilitic  interlocking  habit,  it  sometimes  has  definite 
crystal  form,  with  the  common  hexagonal  sections  from  the  base  and 
rectangular  sections  from  the  prism.  Biotite  is  an  accessory,  but 
tourmaline  has  disappeared  and  does  not  reappear  between  this  point 
and  the  syenite.  Microperthite  occurs  in  intersertal  contact  with  the 
cordierite,  but  is  only  a  rather  rare  accessory.  Besides  ilmenite  or 
titaniferous  magnetite,  the  iron  compounds  include  both  pyrite  and 
pyrrhotite.  The  test  for  the  occurrence  of  the  latter  mineral  and  the 
method  of  its  determination  in  amount  are  the  same  as  has  been  out- 
lined above.  The  strong  basicity  of  the  hornfels  is  noteworthy,  as 
well  as  the  high  content  of  alumina.  The  latter  easily  explains  the 
richness  of  the  rock  in  corundum'*  (see  Table  II,  col.  2). 


a  Cf  J.  Morozewicz,  Experimentelle  Untersuchiingen  iiber  die  Bildung  der  Minerale  im  Magma: 
Tscher.  Mm  und  Petrog.  Mitth.,  Vol  XVIII,  ib98,  p.  5?. 


DALY.]     ^       SPECIMENS    FROM   THE    METAMORPHIC    AUREOLE.  29 

Table  II. — Analyses  of  cordierite-hornf els. 


]. 

2. 

3. 

4. 

SiOa __._^_..,-_. 

TiO^ 

AI2O3                        -               -         

58. 35 
0.87 

21.  30 
0.30 
6.00 
2.10 
0.85 
1.60 
5.63 

«  0. 86 
0.81 

None. 
0.18 

None. 
0.03 

Undet. 
0.19 
0.53 
0.03 
0.13 
0.05 

Trace. 
Strong  tr. 

Trace. 

&0.40 

99.71 

45.30 
1.48 

30.51 
0.24 
8.80 
3.11 
0.90 
1.65 
4.84 

«1.05 
0.26 

Trace? 
0.12 
0.04 
0.04 
0.04 
0.36 
0.96 
0.02 
0.20 
0.03 

Trace. 

Strong  tr. 

? 

CO.  17 

1        55.68 

21.91 
2.63 
6.90 
3.57 
0.89 
1.01 
6.34 

«1.41 

56.88 
20.86 

FegOs 

2.66 

FeO 

MgO 

CaO     -   -     - 

4.54 
3.15 
1.29 

NajO     - 

0.91 

K2O 

7.49 

H2O  above  110=  C 

H2O  below  110°  C. 

«  2. 36 

CO2 

P„0, .                         -     

SC 

CI 

F 

FeSj         -                           -   - 

Fe,SR 

NiO,  CoO 

MnO . 

BaO 

SrO 

LioO  --                   -...     .   .     

CnO 

• 

C 

0-F,Cl 

100. 12 
-    0.02 

100. 34 

100. 14 

0.31 
2.673 

Total  S - 

100. 10 
0.19 
2.835 

Sp.gr _.■ 

a  Loss  on  ignition. 

b  Another  sample  gave  0.03  per  cent  carbon  and  no  OOo- 

0  Another  sample  gave  0.03  per  cent  carbon  and  0.04  per  cent  COo. 

1.  Cordierite-biotite-microperthite-hornfels,  a  phase  of  the  exomorphic  zone 
100  feet  (31  meters)  from  the  contact,  north  side,  Ascutney  Mountain;  analysis  by 
Hillebrand. 

2.  Cordierite-corundum-pleonaste-hor:afels,  taken  from  the  same  cross  section 
of  the  exomorphic  zone  as  No.  1,  25  feet  OS  meters)  frora  the  contact;  analysis  py 
Hillebrand. 

3.  Cordierite-biotite-orthoclase-hornfels,  Schaida,  S.  Carinthia;  analysis  by 
Graber,  Jahrb.  der  K.-k.  geol.  Reichsanst.,  1897,  Vol.  XLVII,  p.  290. 

4.  Cordierite-biotite-plagioclase-hornfels,  M.  Doja,  S.  Carinthia;  analysis  by 
Von  Zeynek.     See  Graber,  ibid.,  p.  290. 


30 


GEOLOGY    OF    ASCUTNEY   MOUNTAIN,   VERMONT.         [bull.209. 


6  feet  and  1  foot  {1.8''meters  and  0.3  meter). — A  specimen  taken  at  a 
point  6  feet  from  the  contact  (spec.  123),  and  another  taken  from  a 
point  only  1  foot  from  it  (spec.  122),  are  very  similar  to  each  other 
and  to  the  phase  just  described.  There  is,  however,  a  decrease  in 
corundnm  and  an  increase  in  pleonaste,  which  now  possesses  perfect 
crystal  form.  The  octahedra  are  excellently  developed,  and  are 
furthermore  interesting,  as  they  show  the  octahedral  cleavage,  which 
is  seldom  seen  in  rock-forming  occurrences.  The  clustering  habit  of 
the  pleonaste  is  strongly  marked  in  both  specimens.  Corundum  often 
appears  as  a  core,  about  which  the  concentration  of  pleonaste  took 
place.  In  other  cases,  the  grouping  is  wholly  in  quartz  crystals  in  a 
manner  similar  to  that  already  noted  for  the  clusters  in  cordierite. 

This  study  of  the  cross  section  of  the  aureole  may  be  summarized 
in  tabular  form  as  follows: 

Summary  of  cross  section  of  series  A  of  vietamorphic  aureole. 


Distance  from 
the  contact. 

Compound  name  of  aureole  phase, 
showing  its  essential  constitution. 

Accessory  and  subordinate  mineral 
constituents. 

Feet. 

Meters. 

600 

183 

Unaltered  argillaceous  phyllite. 

Pyrite,  pyrrhotite  (?),  ilmenite, 
carbonaceous  matter. 

500 

154 

The  same,  crumpled 

Do. 

400 

122 

Crumpled  chloritic  phyllite 

Do. 

300 

91 

Cordierite-quartz-hornf  els 

Biotite,  pleonaste,  epidote,  pyr- 
rhotite, pyrite,  ilmenite,  car- 
bon. 

250 

77 

Pseudo  -  miarolitic   cordieri:;e- 

Pyrite,  pyrrhotite,  ilmenite,  car- 

biotite-quartz-microperthite- 

bon,  tourmaline,  garnet,  horn- 

hornfels. 

blende,  zircon. 

150 

46 

Cordierite  -  garnet  -  pleonaste- 

Pyrite,  pyrrhotite,  corundum,  il- 

quartz-hornfels. 

menite,  carbon. 

100 

81 

Cordierite  -  biotite  -  microperth- 

Pyrite,     pyrrhotite,     pleonaste. 

ite-hornfels. 

carbon  (graphite  ?), ilmenite. 

50 

15.5 

Cordierite  -  pleonaste  -  micro- 

Pyrrhotite,  pyrite.  biotite,  ilme- 

perthite-hornf els . 

nite,  carbon  (graphite  Y) ,  tour- 
maline. 

25 

8 

Cordierite-corundum-pleonaste- 

Biotite,  microperthite,  pyrrho- 

hornfels. 

tite,  pyrite,  ilmenite,  carbon. 

6 
and 

1 

1.8 
and 
0.3 

Cordierite-pleonaste-corundum- 
hornfels. 

Do. 

DALY.]  METAMOKPHISM    OF    THE    PHYLLITES.  31 

SERIES  B    OF    SPECIMENS    FROM    THE    METAMORPHIC  AUREOLE. 

A  second  suite  of  specimens  was  collected  from  a  section  across  the 
metamorphosed  belt  just  south  of  Brownsville.  The  aureole  is  here 
at  least  600  feet  wide,  the  relatively  greater  breadth  being  probably 
due  to  the  proximity  of  the  diorite  as  well  as  the  syenite.  The  effects 
produced  by  the  intrusives  are  practically  the  same  as  in  Series  A. 

600  feet  {183  ineters). — Six  hundred  feet  from  the  contact  along  the 
strike  and  500  feet  across  it  (spec.  136),  garnets,  tourmaline,  corundum, 
and  a  little  cordierite  are  already  found  in  the  j)hyllite,  which,  in  its 
unaltered  phase,  is  more  quai'tzose  than  in  Series  A.  The  quartz 
preserves  much  of  its  original  imjjortance ;  it  is  charged  to  a  remark- 
able degree  with  liquid  inclusions,  often  containing  gas  bubbles. 
Biotite  is  present,  and  is  also  doubtless  inherited  from  the  original 
schist.  Rare  epidote  is  accessory.  No  feldspar  is  recognizable.  Cer- 
tain colorless  grains  with  high  single  and  low  double  refraction  have 
the  appearance  of  andalusite,  but  its  presence  could  not  be  proved 
on  account  of  the  small  size  of  the  individuals. 

300  feet  {91  meters). — Three  hundred  feet  from  the  contact  pleonaste 
appears  as  small  disseminated  grains  for  the  first  time  (spec.  135). 

100  feet  {31  meters). — One  hundred  feet  from  the  contact  the  schis- 
tose structure  is  no  longer  visible  macroscopically,  though  it  appears 
in  the  thin  section  (spec.  134).  The  biotite  assumes  a  concretionary 
rather  than  a  plane-parallel  arrangement.  The  increased  importance 
of  cordierite  and  the  entrance  of  a  few  needles  of  sillimanite  are  the 
other  chief  points  of  difference  from  the  last  locality. 

Jf5  feet  {llf.  meters). — At  45  feet  pleonaste  and  corundum  are  quite 
prominent,  both  in  abundance  and  in  size  of  the  individuals  (spec. 
133).  Sillimanite  increases  in  quantity.  Andalusite  is,  as  before, 
doubtfully  present. 

15  feet  {6  meters). — At  15  feet,  tourmaline  is  added  to  the  list  of 
essentials  (spec.  132).  The  pleonaste  has  two  types  of  aggregation. 
Besides  appearing  on  the  familiar  clusters,  it  accumulates  in  long 
strings,  which  reach  2  millimeters  or  more  in  length  and  have  a  uni- 
form breadth  of  about  0.1  millimeter,  reminding  one  of  the  linear 
development  of  chlorite  in  the  classic  desmosite  (PI.  Ill,  A).  This 
second  kind  of  aggregation  does  not  seem  to  have  any  fixed  relation 
to  single  individuals  of  other  constituents,  and  the  masses  of  pleonaste 
pierce  the  rock  in  all  directions.  Again,  this  mineral  is  inclosed  by 
the  cordierite  in  a  way  not  observed  elsewhere  in  the  aureole.  Numer- 
ous rectangular  grains  of  spinel  may  be  arranged  with  their  longer 
axes  parallel  to  the  chief  axis  of  the  cordierite  host.  Idiomorphic 
cordierite  also  incloses  prisms  of  tourmaline  with  a  similar  orienta- 
tion. Corunduui  is  first  seen  here  to  assume  a  crystal  form.  Silli- 
manite is  a  common  accessory. 

5  feet  {1.6  meters). — A  specimen  (No.  131)  taken  only  5  feet  from  the, 
contact  shows  a  rarity  of  metamorphic  minerals  which  can  only  be 


PLATE    III. 

A,  Hornfels  containing  abundant  pleonaste  (black,  of  high  relief)  and  coriin- 
dtini  (white,  of  high  relief)  in  a  matrix  composed  chiefly  of  cordierite  (a  charac- 
teristic linear  aggregate  of  pleonaste  is  conspicnons) ;  ordinary  light,  X  12.  (See 
p.  39.)  Compare  with  PI.  II,  A  and  B,  illustrating  the  same  phyllite  from  which 
this  hornfels  has  been  j^roduced  by  contact  metamorphism. 

B,  Thin  section  of  quartz-bearing  hornblende-biotite  diorite,  showing  an  apatite 
crystal  inclosing  a  core  of  brown  glass;  ordinary  light,  X  50. 

32 


U.  S.  GEOLOGICAL  SURVEY 


BULLETIN  NO.  209,  PL. 


m; 


(B) 


THE    MERIDEN    GRAVURE    CO. 


DALY.  J  COMPARISON    OF    MET  AMORPHIC    EFFECTS.  33 

» 

explained  as  characterizing  a  phase  of  the  schist  which  was  originally 
less  richly  charged  with  the  argillaceous  material  that  has  elsewhere 
yielded,  under  the  metamorphic  process,  the  minerals  above  mentioned. 
The  schistose  structure  is  reverted  to;  garnet,  pleonaste,  corundum, 
and  tourmaline  are  present,  but  are  very  subordinate  and  confined  to 
the  structure  planes.  Intersertal  microperthite  forms  an  accessory 
it  shows  occasionally  the  Carlsbad  twinning. 

At  the  contact  there  Is  the  same  relative  poverty  in  metamorphic 
products  and  probably  for  a  similar  reason  to  that  adduced  for  the 
last  phase  (spec.  130).  The  rock  is  a  compact  mass  of  quartz,  chlori- 
tized  biotite,  and  pleonaste,  with  ilmenite,  tourmaline,  cordierite,  and 
corundum  as  accessories. 

SERIES  C  OF    SPECIMENS    FROM    THE    METAMORPHIC    AUREOLE. 

A  third  suite  of  specimens  illustrating  the  contact  zone  was  col- 
lected on  the  south  side  of  the  mountain.  The  phenomena  are  essen- 
tially similar  to  those  in  Series  A.  It  would  be  unprofitable  to  give  a 
detailed  description  of  them,  as  it  would  be  in  the  nature  of  repetition. 
The  perfection  of  the  crystallization  of  corundum  is,  however,  worthy 
of  special  note.  Within  the  belt  10  feet  (3  meters)  or  more,  measured 
out  from  the  contact,  it  is  an  abundant  essential  constituent  of  the 
rock.  Both  basal  and  prismatic  sections  of  the  Idiomorphic  and  gran- 
ular crystals  exhibit  cores  of  vivid  blue  in  the  otherwise  colorless  min- 
eral. Each  of  these  cores  is  oriented  with  the  longer  axis  parallel  to  the 
chief  axis  of  the  corundum  individual.  Rotated  over  the  polarizer 
alone,  the  basal  section  shows  no  change  of  color;  in  other  sections 
a  striking  pleochroism,  from  deep  blue  to  colorless,  is  characteristic. 

COMPARISON   OF  THE  METAMORPHIC  EFFECTS. 

The  constant  nature  of  the  metamorphism  is  indicated  not  only  by 
the  correspondence  of  these  three  series  of  specimens,  but  also  by 
many  specimens  collected  from  points  isolated  in  the  contact  zone  and 
not  connected  in  the  serial  arrangement  of  a  cross  section.  Wherever 
the  comparison  could  be  fairly  instituted,  the  alteration  of  the  phyl- 
Utic  rocks  was  seen  to  be  of  the  same  character,  whether  produced  by 
syenite,  granite,  or  gabbro-diorite.  It  is  another  illustration  of  a  now 
familiar  fact — that  so  widely  divergent  intrusive  types  as  granite, 
syenite,  diorite,  diabases,  and  peridotite  may  form  similar  types  of 
hornfels.'* 

The  general  order  of  the  metamorphic  effects  to  be  observed  as  one 
approaches  the  contact  may  be  stated  as  follows : 

At  500  feet  or  less  from  the  contact  there  begins  to  be  apparent  a 
distinct  loss  of  schistosity  in  the  phyllite.  The  rock  gains  in  massive- 
ness  and  in  specific  gravity.     The  extent  of  these  changes,  as  of  all 

«Cf.  Lacroix,  Comptes  Rendus,  18  fevr.,  1895. 

Bull.  209—03 3 


34  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.^       [bull.209. 

the  others,  is  manifestly  controlled  by  the  nature  of  the  particular 
phyllitic  band  studied.  The  dominant  mineralogical  essential  of  the 
aureole  is  cordierite,  which  appears  suddenly  and  abundantly  in  the 
outer  part  of  the  aureole.  Second  in  importance  to  that  mineral  is 
j)leonaste,  assuming  greater  quantitative  importance  and  greater  size 
and  perfection  of  crystal  form  in  its  individuals  as  the  contact  is 
neared.  Metamorphic  biotite  is  likewise  abundant.  Its  increase 
means,  as  a  rule,  a  decrease  of  pleonaste  in  that  particular  phase. 
Corundum  behaves  like  the  spinel  in  its  progressive  development 
toward  the  contact,  but  appears  later  in  the  section.  Sillimanite  is 
confined  to  the  inner  part  of  the  zone,  but  is  never  abundant.  Garnet, 
tourmaline,  and  andalusite  (?)  are  sporadic,  appearing  and  disappear- 
ing irregularly  in  the  cross  section,  though  more  likely  to  appear  at 
its  inner  extremity. 

Feldspathization  characterizes  the  aureole  as  far,  at  least,  as  300 
feet  (91  meters)  from  the  intrusive.  It  is  higlil}^  probable,  however, 
from  the  evidence  of  the  hornfels  analyses  and  of  the  microscopic 
examination,  that  the  transfer  of  material  from  the  stock  magmas  to 
their  country  rock  is  but  subordinate  in  quantity.  The  mere  heat  of 
the  intrusion  would  doubtless  have  been  sufficient  to  iDroduce  some 
of  the  more  important  new  minerals.  Cordierite  and  spinel  in  abun- 
dance have  been  formed  in  coal-bearing  mica-slates  (schists)  through 
the  melting  up  of  the  slates  during  the  combustion  of  the  coal.* 
These  minerals  have  also  been  observed  to  be  the  result  of  the  altera- 
tion of  the  micaceous  inclusions  caught  up  by  volcanic  flows.  The 
question  as  to  just  how  much  material  has  been  added  to  the  schists, 
either  as  alkaline  silicate  or  in  other  form,  can,  however,  not  be  satis- 
factorily and  finally  discussed  until  the  same  phyllitic  band  has  been 
followed  across  the  aureole  and  analyses  been  made  from  that  band 
where  it  is  unaltered  as  well  as  where  it  has  been  strongl}^  metamor- 
phosed. So  far  it  has  proved  apparently  impossible  to  follow  any  one 
band  across  the  whole  zone.  The  only  analysis  j^et  made  of  the  unal- 
tered schist  relates  to  a  quartzitic  phase  outcropping  at  a  distance 
from  the  mountain  (see  Table  I).  11"  all  tlie  soda  and  potash  in  even 
that  i^hase  were  to  enter  into  the  jn'oper  combinations  with  the  alumina 
and  silica  as  much  as  ten  jyev  cent  or  thereabouts  of  alkaline  feldspar 
would  result.  The  belief  that  feldspathization  has  really  occurred 
would  thus  be  more  strongly  upheld  by  the  peculiar  nature  of  the 
actual  microperthitic  intergrowth,  so  similar  in  every  respect  with  the 
feldspar  of  the  adjacent  syenite,  and  by  the  nonoccurrence  of  that 
intergrowth  in  the  phjdlite  outside  the  aureole,  rather  than  by  the  evi- 
dence derived  from  the  analysis  of  an  unaltered  phyllite  more  argil- 
laceous than  the  quartzitic  phase. 

A  list  of  the  metamorphic  constituents  of  the  aureole,  arranged  in 
the  relative  order  of  abundance  as  nearly  as  maj^  be  by  mere  inspection 
of  thin  sections,  may  complete  this  brief  summary. 


wLacroix,  Les  enclaves  des  roches  volcaniques,  Macon,  1893,  p.  577. 


DALY.]  SUMMARY    OF   METAMORPHIC    EFFECTS.  35 

List  of  metamorphic  constituents  of  the  aureole. 

In  the  limestones. 
Epidote. 
Grosstilarite. 
Scapolite. 
Hornblende. 
Titanite. 

In  the  phyllites. 
Cordierite. 
Biotite. 
Pleonaste. 
Corundum. 
Lime-iron  garnet. 
Sillimanite. 
Pyrrhotite. 
Pyrite. 
Tourmaline. 
Andalnsite  (?). 
Graphite  (?). 


Microperthite 

Quartz 

Brown  hornblende 

Biotite 


Introduced  sub- 
stance of  the  "pseu- 
do-miaroles "  and 
intersertal  areas. 


There  are  strong  resemblances  between  this  list  and  those  made 
out  by  G.  H.  Williams  at  the  Cruger's  section  (diorites  of  the  Cort- 
landt  series  metamorphosing  biotite-muscovite  schists),*  and  by  Teller 
and  Von  John  in  the  Tyrol  (norites  and  diorites  cutting  phyllites  and 

gneisses).^ 

SUMMARY. 

The  schists  display  unequal  effects  of  contact  metamorphism  where 
they  lie  in  contact  with  the  intrusive  bodies.  As  was  to  be  expected, 
the  gneisses  are  much  the  more  stable  and  exhibit  little  mineralogical 
change  even  close  to  the  eruptive  contacts,  but  the  abundant  argil- 
lacous  material  of  the  phyllites  has  been  extensively  recrystallized 
into  a  well-defined  zone  of  hornfels.  Cordierite,  pleonaste,  biotite, 
garnet,  corundum,  and  epidote  form  the  chief  secondary  minerals  thus 
developed.  The  limestone  bands  have  richl}'  yielded  grossularite  and 
epidote  in  the  same  contact  zone.  Repeated  occurrences  of  intersti- 
tial microperthitic  feldspar  lead  to  the  conclusion  that,  during  the 
intrusion  of  the  syenites  and  granite,  feldspathization  of  the  phyllitic 
country  rock  has  taken  place. 


« Am.  Jour.  Sci.,  3d  series,  Vol.  XXXVI.  1888,  p.  254. 

6  Jahrbuch  K.-k.  geol.  Reichsanstalt,  Vol.  XXXII,  1883,  pp.  655  and  S. 


CHAPTER    IV. 

THE  ERUPTIVE  ROCKS. 

GENERAL  TABLE  AND  CORRELATION. 

It  was  thus  into  a  series  of  tilted  metamorphosed  sediments  that  the 
irruptions  with  whicli  we  are  here  particulai-ly  concerned  began  (see 
PL  VII).  The  variety,  as  well  as  the  relative  ages  of  the  resulting 
rock  bodies,  is  indicated  in  the  accompanying  table,  which  gives  a  sum- 
mary statement  of  the  succession,  from  the  oldest  to  the  youngest 
intrusive: 

Table  III. — Rock  bodies  resulting  from  the  irruptions. 
(From  oldest  to  youngest.) 

A.  Basic  stock  of  five  chief  phases,  viz: 

a.  Augite-gabbro. 

b.  Hornblende-biotite-augite-gabbro. 

c.  Biotite-hornblende-diorite. 

d.  Biotite-aiigite-hornblende-diorite. 

e.  Orthoclase-microperthite-bearinghornblende-biotite-diorite  (containing 

basic  segregations)  =  acid  essexite. 
This  stock  is  cut  by — 

1.  Reticulate  intrusions  (forming  intrusion-breccias)  of  augite-biotite-diorite 

with  and  without  essential  hornblende. 

2.  Dikes  of  "  windsorite."'  the  alkaline  equivalent  of  granodiorite. 

3.  Nordmarkite  porphyry  stock-like  dike  of  Little  Ascutney  (bearing  basic 

segregations  and  cut  by  Nos.  6  and  7). 

4.  Main  stock  (B)  of  Ascutney  Mountain  and  its  apophyses. 

5.  Pulaskite  (quartzless  biotite-nordinarkite)  stock  of  Pierson  Peak. 

6.  Hornblende-ijaisanite  dike  (cut  by  camptonites). 

7.  Caniptonite  dikes. 

8.  Diabase  diiies  (?) . 

B.  Main  stock  of  Ascutney  Mountain,  of  four  chief  phases,  viz: 

f.  Hornblende-biotite-nordmarkite  of  granitic  structure   (bearing  basic 

segregations) . 

g.  Hornblende-biotite-atigite-nordmarkite  of  porphyritic  structure  (bear- 

ing basic  segregations), 
h.  Alkaline  granites  without  essential  bisilicates. 
i.  Monzonite. 
This  stock  is  cut  by— 

9.  Hornblende  paisanite  dikes. 

10.  Camptonite  dikes. 

11.  Diabase  dikes. 

12.  Common  muscovite  aplite. 

13.  Stock  C. 

C.  Stock  of  alkaline  biotite-granite  (bearing  basic  segregations) . 

36 


DALY.]  CORRELATION    OF   THE    INTRUSIVE    ROCKS.  37 

A  general  correlation  of  these  Intrusives  in  terms  of  eruptive  periods 
and  composition  maj^  be  made  in  the  form  of  a  second  table : 

First  eruptive  period A  (a.  b,  c,  d,  e)  (  r^  ■,-,       -,■     ... 

^■^,.            ..,                                           '-,     h  Gabbro-dioritic  magma, « 
Second  eruptive  period 1  and  2  ) 

Third  eruptive  period B  (f.  g,  h,  i),  3  and  5  )  c.       ■^^ 

-r^       ,.            ,.            ■    ■,  „       .,  ^  -  Sy  em  tic  magma. 

Fourth  eruptive  period 6  and  9  ) 

Fifth  eruptive  period C  and  12  Granitic  magma  and  aplite. 

Sixth  eruptive  period _ 7  and  10,  8  and  11  Lamprophyres. 

The  reference  of  the  individual  intrusives  to  the  different  magmas 
or  their  derivatives  is  indisputable  except  in  the  case  of  No.  2,  which 
is  intermediate  both  in  composition  and  in  geological  age  between  the 
Basic  stock  and  the  Main  syenite  stock.  The  reference  to  the  differ- 
ent eruptive  periods  as  stated  in  the  table  is  to  some  extent  arbitrary. 
The  stocks  A,  B,  and  C  certainly  followed  one  another  in  the  order 
named.  As  the  map  shows,  the  conduit  through  which  the  substan- 
tial contributions  to  the  whole  mass  were  made  migrated  from  west 
to  east.  The  intimate  field  relations  of  A,  1  and  2,  seem  to  show  that 
all  three  antedate  the  syenites.  Nos.  1  and  2  cut  A  and  are  probably 
older  than  B  or  its  equivalent.  Nos.  3  and  5  are  con-elated  with  B 
on  the  ground  of  close  mineralogical  and  structural  similarity;  they 
are  clearly  older  than  Nos.  6  and  9,  which  were  probably  not  strictly 
contemporaneous.  Though  stock  C  also  cuts  B,  it  is  probably  not 
contemporaneous  with  No.  6  or  No.  9.  It  is  probable,  though  not 
proved,  that  stock  B  is  older  than  the  lamprophyres,  which  are  cer- 
tainly younger  than  all  the  syenitic  intrusions. 

In  all  the  stocks  and  in  many  dikes  of  the  area,  nodular  masses  of 
segregational  basic  materials  occur.  They  are  most  common  in  the 
syenites,  less  so  in  the  alkaline  granite  stock,  and  comparatively  rare 
in  the  diorities.  These  nodules  were  early  noted  and  described  by 
Edward  Hitchcock  and  others,  and  remarked  for  their  extreme  abun- 
dance. They  will  be  treated  of  in  some  detail  in  the  following  pages 
in  connection  with  the  petrographical  description  of  their  parent 
rocks; 

It  will  be  seen  that  among  the  petrographical  methods  emploj^ed, 
those  for  the  determination  of  the  feldspars  have  been  in  the  most 
constant  requisition.  In  order  to  avoid  repetition  and  a  certain  degree 
of  monotony  in  the  description,  the  actual  readings  on  which  the 
determinations  of  the  various  species  have  been  based  are,  as  a  rule, 
not  given  in  the  text.  In  general,  several  independent  methods  have 
been  used  for  each  determination.  Examples  of  these  are  noted  in  the 
discussion  of  the  rocks  comj)osing  the  largest  stocks  examined.  In 
the  comi^ound  names  of  some  of  the  rock  types,  as  well  as  in  the  tables 
of  mineralogical  composition,  the  mineral  constituents  are  entered  in 
the  order  of  decreasing  importance  in  their  resi)ective  rocks. 

«A  magma  intermediate  in  average  composition  between  gabbi'o  and  diorite.    The  adjective 
is  not  derived  from  the  term  "  gabbro-diorite,"  as  iised  by  G.  H.  Williams  or  by  Tornebohm. 


38  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,    VERMONT.         [Bri.L.209. 

GABBROS,  DIORITES,  ANB  REI.ATED  ROCKS. 

BASIC  STOCK;  GABBRO  AND   DIORITE  PHASES. 

The  oldest  of  the  intrusives  illustrates  the  common  characteristic 
of  basic  stocks  in  exhibiting  considerable  variation  of  composition, 
structure,  color,  and  texture  in  its  rock  types.  A  large  number  of 
thin  sections  were  made  from  specimens  collected  in  all  parts  of  the 
stock.  They  show  that  its  material  occurs  in  the  form  of  five  differ- 
ent phases,  repeatedly  occurring  in  more  or  less  typical  form,  and 
connected  with  one  another  by  transitions.  All  five  seem  to  have  been 
differentiated  from  the  product  of  a  single  intrusion.  That  Ave  have 
here  to  do  with  an  eruptive  of  a  truly  exotic  nature  is  abundantly 
proved  at  almost  any  point  of  the  schist  contact.  Both  to  the  north 
and  to  the  south  of  Little  Ascutney  excellent  examples  of  intrusive 
dikes  and  sills,  plainly  apophysal  from  the  massive  rock,  are  to  be 
found  cutting  the  gneisses.  The  horses  of  schistose  rock  are  so  abun- 
dant at  many  contacts  as  to  make  up  veritable  flow  breccias,  and  frag- 
ments are  occasionally  found  several  hundred  yards  from  the  contact. 

Phase  d  is  the  one  rock  type  from  the  Basic  stock  which  has  been 
chemically  analyzed.  It  is  somewhat  more  acid  than  the  average 
type,  but  it  was  selected  on  account  of  its  relative  freshness.  The 
specimen  analyzed  (Table  IV,  col.  1)  was  taken  from  some  blasted 
ledges  about  100  yards  north  of  Mr.  Pierson's  house,  on  the  notch  road 
between  the  two  Ascutneys.  It  is  a  fairl}^  coarse-grained  dark  bluish- 
gray  quartz-diorite  of  tj^pical  hypidiomorphic  granular  structure,  in 
which  the  essential  dark-colored  silicates  are  biotite  and  a  diopsidic 
augite  with  subordinate  brown  hornblende  (spec.  35), 

The  feldspar  of  the  rock  is  almost  alwaj^s  multiple-twinned,  follow- 
ing the  albite  law,  more  rarely  the  pericline  law.  The  ver}^  common 
association  of  albite  and  Carlsbad  twinning  in  the  slide  makes  it  pos- 
sible to  determine  the  feldspar  with  a  high  degree  of  accuracy.  In 
addition,  Becke's  method  of  differential  single  refraction,  the  reading 
of  extinctions  on  cleavage  pieces  and  on  sections  cut  parallel  to 
the  bissectrices,  and  the  principle  of  equal  illumination  in  the  zoned 
individuals  agreed  well  in  establishing  the  average  mixture  of  the 
soda  and  lime  molecules  as  one  slightly  more  acid  than  the  basic  oli- 
goclase.  Aba  -^^i-  ^^^  single  individuals  maj^  vary  from  the  acid 
oligoclase  Abj  Aiij  to  bj'townite  Ab,  An4.  Zoned  crystals  are  com- 
mon. The  cores  range  from  Abj  An4  to  Abj  Auj,  with  an  average  close 
to  the  andesine  Ab4  Aiig,  while  the  outer  zones  seem  to  be  invariably 
more  acid,  with  an  upper  limit  at  the  acid  oligoclase  Ab^  Auj. 

The  tolerabl}^  high  percentage  of  potash  in  the  analysis  leads  one  to 
suspect  a  potash  feldspar,  but  diligent  search  has  so  far  failed  to 
establish  the  i^resence  of  either  ortlioclase  or  microcline.  The  refrac- 
tion of  the  rare  untwinned  individuals  was  carefully  compared  in  a 
number  of  slides  with  the  refraction  of  quartz  and  undoubted  plagio- 


DALY.]  PHASES    OF    THE    BASIC    STOCK.  39 

clase,  and  showed,  often  with  the  corroboration  of  convergent  light, 
that  these  untwinned  individuals  are  likewise  soda-lime  felds^jars 
averaging  basic  oligoclase.  The  determination  of  orthoclase  in  the 
rock  powder  is  impossible  because  the  basic  oligoclases  and  acid 
andesines  occur  in  such  profusion.  The  potash  of  the  analysis  must, 
then,  be  referred  primarily  to  the  biotite,  and,  in  a  notable  degree,  to 
isomorphic  mixture  with  the  soda-lime  feldspar.  That  there  must  be 
some  potash  outside  of  the  biotite  is  not  to  be  doubted,  for,  granting 
the  high  proportion  of  10  per  cent  of  potash  in  the  biotite,  and  credit- 
ing that  mineral  with  all  the  oxide,  there  must  be  at  least  23.4  ]3er 
cent  biotite  in  the  rock.  Inspection  shows  that  even  tliat  minimum 
proportion  of  the  mica  is  not  rex3resented,  although  it  is  next  the  feld- 
spar in  abundance.  It  has  the  usual  properties  of  biotite  from  normal 
granitic  rocks — powerful  pleochroism  and  absorption  in  brown  and 
yellow  tones,  extremelj^  small  optical  angle,  and  parallel  extinction. 

The  augite  is  crystallized  both  alone  and  in  the  form  of  intergrowths 
with  hornblende.  In  both  cases  the  habit  of  the  mineral  is  that  of 
common  diopsidic,  colorless  to  pale  greenish,  allotriomorphic  indi- 
viduals from  1  to  o  millimeters  in  diameter.  A  third  cleavage  parallel 
to  (100)  occurs  in  a  few  sections.  Twinning  parallel  to  (001)  is  not 
rare.  Pleochroism  is  absent.  The  alteration  is  largely  confined  to 
chloritization,  but  paramorphic  changes  to  a  uralitic  amj)hibole  are 
common. 

The  brown  hornblende  almost  invariabl}^  forms  intergrowths,  either 
irregular  or  oriented  in  parallel  fashion  with  augite.  In  all  cases  the 
mineral  is  doubtless  X)rimary.     The  pleochroism  is  as  follows : 

a.  Pale  grayish  j'ellow. 

b.  Greenish  brown  with  a  tinge  of  olive  (medium  absorption). 

c.  Brown  (medium  to  strong  absorption). 
c>b>a. 

The  prismatic  angle  is  55°  o2'  (average  of  measurements  ou  eight 
cleavage  pieces).  The  extinction  on  (110)  is  al)out  12°  30',  and  on 
(010)  about  15°.  It  is  seen  that  the  mineral  belongs  to  a  common 
variety  of  araphibole. 

Quartz  forms  allotriomorphic,  cementing  grains  which  represent 
the  last  stage  in  the  crystallization  of  the  rock.  It  is  never  present 
in  large  amount.  Gas  and  liquid  inclusions,  often  simulating  nega- 
tive crystals  in  form,  are  common,  particularly  in  the  coarser-grained 
specimens  of  the  rock. 

Apatite  needles  and  larger  crj^stals  are  abundant.  A  characteris- 
tic feature  of  this  accessory  is  the  common  inclusion,  in  the  form  of 
elongated  cores,  of  isotropic,  probably  glass,  bodies  of  a  deep-brown 
color  (PI.  Hi,  ^.) 

Ilmenite  or  titaniferous  magnetite  and  iDrimary  titanite  with  weak 
pleochroism  are  important  accessories.  Large  zircon  crystals  and 
occasional  grains  of  pyrite  complete  the  list  of  accessories. 

The  structure  is  hydiomorphic-granular.      The  feldspar  is  often 


40  GEOLOGY    OF    ASCUTNEY    MOUNfAUSt,   VEEMONT.         [bull.  209. 

idiom  Orphic  against  both  augite  and  quartz.  The  biotite  is  always 
idiomorphic  against  quartz,  rarely  against  feldspar,  and  may  inclose 
all  the  other  constituents  in  poikilitic  fashion.  The  augite  is  usually 
intersertal  with  reference  to  the  feldspar  and  incloses  all  the  accesso- 
ries. The  sequence  of  crystallization  apj)ears  to  have  been  (in  order 
from  the  oldest  to  the  youngest  mineral)  as  follows : 

Apatite. 

Titanite,  zircon  and  ilmenite. 

Feldspar.  | 

Angite  (and  hornblende) .  [    Nearly  contemporaneotis. 

Biotite.  J 

Qnartz. 

The  composition  of  the  biotite,  augite,  and  hornblende  not  being 
known,  it  is  not  possible  to  calculate  the  analysis.  The  analysis 
agrees  with  the  microscopic  examination  in  placing  this  rock  decid- 
edly in  the  class  of  true  diorites,  as  a  hiotiie-augite-hornblende-diorite. 
For  ease  of  reference,  columns  2  and  3  are  entered  in  Table  IV  in 
order  to  show  the  similarity  between  our  rock  and  a  fair  average 
diorite  and  also  the  limits  of  chemical  variation  which  can  be  found 
in  a  list  of  typical  analyses  from  that  rock  group.  From  its  field 
associations  one  might  expect  the  Basic  stock  to  have  given  a  higher 
proportion  of  alkalies  in  its  average  phase.  For  this  reason  column 
4  has  been  added  to  point  the  dissimilarity  between  the  Ascutnej^ 
rock  and  essexite,  the  alkaline  type  nearest  to  it  in  general  habit. 
The  absence  of  monoclinic  feldspar  and  of  olivine  among  the  con- 
stituents, and  the  relatively  low  proportion  of  soda  and  of  ferric  iron 
compared  to  the  essexite,  serve  to  dispel  the  a  priori  notion  that  the 
alkaline  stocks  on  the  east  should  be  accompanied,  in  their  common 
conduit,  by  an  intrusive  of  essexitic  or  allied  alkaline  habit  if  that 
intrusive  were  to  be  a  plagioclase  rock  of  basic  character. 

Phase  c. — A  second  phase  intimately  allied  to  tlie  first,  both  in  field 
relations  and  lithological  characters,  is  represented  in  a  long  series  of 
outcrops  lying  southeast  of  Mr.  Pierson's  house  on  the  Notch  road. 
Two  sj)ecimens  of  the  brownish-gray,  medium-grained  rock  were 
selected  at  a  point  500  yards  distant  from  the  house,  and  proved  to 
be  a  normal  hiotUe-hornblende-diorite,  with  structure  and  accessories 
similar  to  those  in  phase  d  (spec  32).  The  feldspars  generally  show 
two  zones  of  growth ;  the  cores  average  a  labradorite  between  Ab^  Auj 
and  Abj  Aug,  the  narrower  mantling  zone  averaging  the  oligoclase  Abg 
Auj.  The  average  soda-lime  feldspar  is  probably  close  to  that  of  the 
analyzed  phase.  The  chief  difference  from  the  latter  rock  lies  in  the 
replacement  of  the  augite  by  an  idiomorphic  brown  hornblende  of 
much  deeper  absorption  than  that  characterizing  the  intergrown  horn- 
blende of  phase  d.  Here  the  scheme  of  absorption  is :  a.  Yellow,  b. 
Deep  brown,  with  a  suggestion  of  olive-green,  c.  Deep  chestnut- 
brown.     c>b>aorc  =  b>a. 

One  or  two  large  subidiomorphic   individuals  of   nearly  colorless 


DALY.] 


PHASES    OF   THE    BASIC    STOCK. 


41 


augite  have  been  found  in  the  slides.  In  each  case  a  broad  mantle  of 
the  deep-brown  hornblende  surrounds  the  augite  in  x)arallel  inter- 
growth.  The  biotite  is  of  the  same  nature  as  in  phase  d,  but  much 
less  abundant  and  seldom  poikilitic. 

Table  IV. — Analyses  of  diorites  and  essexite. 


1. 

■  -i 

3. 

4. 

SiOj-.                      _   -               

53.13 

16.35 
3.68 
6.03 
4.14 
7.35 
3.65 
2.34 
0.88 
0.35 
0.07' 
3.10 
0.03 
0.89 
0.09 
0.03 
0.34 

Trace. 
0.17 
0.04 

Trace? 

Trace. 

56. 53 
16.31 
4.38 
5.93 
4.33 
6.94 
3.43 
1.44 

1        1.03 

53.00-63.80 
13. 41-18. 00 
0.77-  7.43 
2.41-12.84 
2.03-  8.03 
4. 99-  8. 98 
3.31-  4.65 
0.44-  2.37 

0.16-  2.24 

47.94 

ALO, 

17.44 

FeaOs 

6.84 

FeO 

6.51 

MgO                                         

2.02 

CaO 

7.47 

Na^O: 

5.63 

K2O     . 

2.79 

H^O  above  110"  C 

H2O  below  110'  C---. 

2.04 

CO2 

TiO. 

0.35 

0.03-  1.10 

0.20 

ZrO^- -     _._   _ 

P2O5 

0.40 

0.17-  1.06 

1.04 

CI 

F 

FeS^ 

NiO,  CoO 

MnO 

0.14 

BaO 

SrO 

Li,0             

100. 33 
0.03 

100. 98 

99.93 

0=F,  C1-- - 

Totals 

100. 30 
0.13 
3.936 

Sp.  ^T         _        

1.  Biotite-augite-liornblende-diorite,  Basic  stock,  Ascutney  Mountain.    Analysis 
by  Hillebrand. 

2.  Average  of  a  series  of  16  typical  diorites,  compiled  by  Brogger,  Die  Eruptiv- 
gesteine  des  Christianiagebietes,  Vol.  II,  1895,  p.  37. 

3.  Limits  of  variation  in  the  above-mentioned  16  analyses. 

4.  Classic  essexite,  Salem  Neck,  Salem,  Mass.     Analysis  by  Dittrich. 

Phase  b. — At  various  points  in  the  stock,  especially  west  and  south 
of  Pierson  Peak  and  near  the  crest  of  Little  Ascutne^^  ridge  (spec.  61), 


42       GEOLOGY  OV   ASCUTNEY  MOUNTAIN,  VERMONT.    [buu.209. 

the  rock  becomes  coarser  than  in  either  of  the  two  phases  just 
described  and  shows  a  distinct  difference  of  composition  from  either. 
At  the  same  time,  repeated  examination  of  ledes  in  the  field  could 
discover  no  difference  of  age  among*  the  three.  This  third  phase  is 
dark  colored  and  remarkable  for  its  richness  in  bisilicates,  which 
have  a  strong  poikilitie  habit.  Augite  and  biotite  of  the  general  char- 
acter of  those  minerals  in  jjhase  d,  and  large  independent  crystals 
with  augite  intergrowths  of  brown  hornblende  similar  to  that  in  phase  c, 
are  the  essential  dark-colored  constituents.  The  biotite  has,  how- 
ever, an  optical  angle  considerably  greater  than  elsewhere  observed 
in  the  stock;  it  was  measured  and  found  to  be  a  few  minutes  more 
than  r. 

A  long  series  of  feldspar  determinations  accorded  with  one's  first 
impression  of  the  rock  in  stnd^'ing  the  hand  specimen,  that  it  belongs 
to  a  phase  much  more  basic  than  c  and  d.  The  prevalence  and  large 
size  of  the  Carlsbad  albite  twins  and  the  uniform  behavior  of  the 
feldspar  enables  us  to  state  conclusively  that  the  average  feldspar  in 
this  phase  is  close  to  basic  labradorite,  Ab.jAug,  with  a  narrow  range 
above  and  below  the  acidity  of  that  mixture.  Primary  quartz  is 
entirely  absent.  Nearly  colorless  titanite  is  especially  abundant,  both 
alone  and  surrounding  the  large  ilmenites  after  the  manner  of 
leueoxene. 

The  basicity  of  this  phase  unquestionably  places  it  among  the  gab- 
bros,  and  it  may  be  called  a  liornblende-hiotite-augife-gabbro,  not- 
withstanding the  absence  of  true  diallage  among  the  constituents.'* 
It  probably  rivals  i)hase  d  in  the  amount  of  surface  covered  in  the 
stock. 

Phase  a. — In  the  fields  west-southwest  of  Pierson  Peak  a  fourth 
variant  of  the  rock  outcrops  in  the  form  of  an  unusirally  coarse- 
grained type  (spec.  112).  It  is  more  feldspathic  than  phase  h  and 
hence  of  a  lighter  color.  Biotite  and  hornblende  have  almost  com- 
pletely disappeared,  the  former  being  a  rare  accessory,  the  latter 
forming  occasionally  a  mantle  about  augite.  The  light  reddish-brown 
feldspar  is  again  very  uniform  and  averages  the  basic  labradorite, 
AbiAuj.  The  usual  accessories  ai-e  present  excepting  quartz.  The 
structure  is  the  hypidiomorphic  granular,  but  on  account  of  the  inter- 
sertal  relation  of  the  augite  to  the  feldspar,  it  assumes  the  special 
habit  of  diabase.  The  poikilitie  nature  of  the  augite  is  very  striking. 
The  phase  has  the  composition  and  other  characters  of  a  typical 
diabase  excepting  irr  its  geological  occurrence.  It  may  be  called  an 
auyde-gabhro,  though  diallage  is  here,  too,  wanting. 

Plidse  e.  —  [•"'inally,  a  fifth  j)hase  remains  to  be  noted,  which  is  not 
important  on  account  of  the  amount  of  area  covered  by  it  in  the  stock 
as  a  whole,  but  which  merits  particular  attention  on  account  of  its 


"We  can  not  biitagree  with  Judd  (Quart.  Jour.  Geol.  Soc.Vol  XLII,1886)  and  Lacroix  (Bull. 
Sferv  cartt  g^eol  France,  No.  67,  Vol.  X,  18S9,  p.  27)  in  regarding  it  a.s  indifferent,  for  purposes 
of  nomenclature,  whether  the  ijyioxene  of  a  gabbro  possess  the  diallagic  structure  or  not. 


DALY.]  BASIC    SEGEEGATIONS.  48 

forming  a  transitional  rock  type  between  the  true  gabbros  and  dio- 
rites  on  the  one  hand  and  the  alkaline  rocks  of  the  region  on  the 
other.  This  phase  was  discovered  just  north  of  the  contact  between 
the  stock  and  the  great  syenite-porphyry  dike  of  Little  Ascutney, 
and  opposite  the  middle  of  that  dike.  The  rock  is  fairly  fresh  and 
tolerably  coarse  grained,  and  is  rich  in  feldspar,  brown  hornblende, 
and  biotite  (spec.  59).  Augite  forms  in  a  few  rare  instances  small 
cores  of  hornblende  intergrowths.  The  accessories  common  to  all  the 
phases  are  present,  and,  in  addition,  some  free  interstitial  quartz. 
But  the  feldspars  are  in  great  contrast  to  those  so  far  noted  as  occur- 
ring in  the  stock.  Plagioclase,  averaging  near  the  andesine  AbgAug 
is  dominant;  oligoclase  ranging  between  AbgAn^  and  AbjAnj  is  com- 
mon. The  plagioclase  is  sometimes  surrounded  by  a  mantle  of  ori- 
ented microperthite.  What  is  still  more  noteworthy  is  the  existence 
of  much  free  orthoclase  and  microperthite  alongside  the  triclinic 
feldspar.     The  order  of  crystallization  is  as  follows: 

Apatite. 

Titanite,  zircon,  and  ilmenite. 

Angite,  hornblende,  and  biotite. 

Oligoclase-andesine. 

Orthoclase  and  microperthite. 

Quartz. 

The  structure  of  the  rock  is  the  usual  hypidiomorphic  granular. 
Its  true  relation  to  the  diorites  is  indicated  if  we  call  it  an  orthoclase- 
microperthite  -  hearing  lioriiblende  -  biotite  -  diorite.  Yet  the  rock  is 
clearly  allied  to  a  somewhat  acid  form  of  essexite. 

BASIC    SEGREGATIONS. 

In  phase  e,  at  the  locality  indicated,  basic  segregations  were  spar- 
ingly found  (spec.59a).  These  are  of  distinctly  darker  color  than  the 
parent  rock,  and  both  macroscopically  and  microscopically  are  seen 
to  be  finer  grained.  They  are  roundish  in  form  and  average  about  1 
inch  (2. 6  mm. )  in  diameter.  The  boundary  between  nodule  and  parent 
rock  is  not  definite ;  they  merge  into  each  other  in  a  gradual  way.  The 
nodules  are  essentially  composed  of  j)lagioclase,  hornblende,  and  bio- 
tite grouped  with  a  panallotriomorphic  structure.  The  feldspar  is 
zoned;  the  most  acid  zone  is  the  oligoclase,  Ab^Auj,  and  the  average 
feldspar  is  near  labradorite,  AbjAuj.  No  certain  orthoclase  or  micro- 
perthitic  feldspar  could  be  identified  in  the  slide.  The  dark  constit- 
uents have  the  usual  properties  of  those  minerals  m  this  stock.  The 
augite  is  more  abundant  here  than  in  the  parent  rock,  though  again 
generally  it  occurs  in  the  form  of  intergrowths  with  the  hornblende. 
Zircon  is  very  rare,  but  apatite  unusually  abundant.  A  little  inter- 
stitial quartz  is  accessory. 


44  GEOLOGY    OF   ASCUTNEY    MOUNTAIN,   VERMONT.         [bull. 209. 

Table  V. — Analysis  (by  Hillehrand)  of  hornblende-biotite-diorite  nodule. 

Per  cent. 

SiO^ - 55.28 

AI2O3 17.33 

Fe^Og 1.54 

FeO 6.23 

MgO 2.69 

CaO 5.60 

Na^O 5.42 

K^O 2.12 

H^OabovellO"  C 0.71 

H,ObelowllO°C _• 0.20 

CO2 0.04 

TiO, 1.64 

ZrO, Trace. 

PA : 0.73 

CI 0.07 

F 0.28 

FeS^ - 0.07 

MnO 0.24 

BaO :^ 0.06 

SrO   Faint  trace. 

LijO Trace. 

100. 15 
0=F,C1 --- 0.13 

100. 02 

Totals 0.038 

Sp.gr .       2.822 

The  analysis  of  one  of  these  nodules  agrees  with  the  microscopic 
diagnosis  (except  in  the  matter  of  structure)  in  placing  it  in  classi- 
fication among  the  hornblende-biotite-diorites  (Table  V).  The  high 
soda  relates  the  nodule  to  essexite.  It  is  more  basic  than  its  host, 
though  more  acid  than  the  diorite  analyzed  (j)hase  d.)  The  high 
fluorine  is  again  noteworthy. 

DIORITIC  DIKES  CUTTING  THE  BASIC  STOCK. 

Following  the  consolidation  of  the  Basic  stock  the  same  magma 
which  it  represents  seems  to  have  been  erupted  a  second  time,  and  as  a 
result  we  have  networks  of  interlacing  dikes  in  various  parts  of  the 
stock.  These  are  oftentimes  so  numerous  as  to  give  the  bare  ledges 
the  appearance  of  mosaics  on  a  large  scale.  Occasionally  the  younger 
intrusive  has  so  extensively  displaced  the  older  as  to  form  miniature 
stock-like  bodies  sending  out  apophyses  into  the  coarser  rock  and 
inclosing  horses  of  the  latter.  The  mosaics  are  thus  intrusion-brec- 
cias or  flow-breccias.  The  younger  intrusives  cut  with  apparent 
indifference  both  the  gabbroitic  and  the  dioritic  phases  of  the  stock. 

In   color,  mineralogical  and   chemical  composition,  and   even  in 


DALY.]  DIKES    CUTTING    THE    BASIC    STOCK.  45 

structure,  the  dikes  are  closely  allied  to  the  diorite  analyzed.  In 
the  fields  southeast  of  Pierson  Peak  both  the  younger  and  older 
rocks  are  augite-biotite-hornblende-diorites  (spec.  147).  Within  a 
hundred  yards  a  second  network  of  dikes  is  characterized  by  the  com- 
plete absence  of  hornblende  and  by  a  remarkably  perfect  zonal  struc- 
ture in  its  feldspar  (spec.  14oa).  The  great  range  in  acidity  of  these 
feldspars  was  not  found  equaled  in  any  other  rock  of  the  whole  region. 
By  the  method  of  equal  illumination,  checked  by  the  behavior  of  each 
zone  in  convergent  light,  it  could  be  proved  that  the  core  of  such  a 
feldspar  may  be  a  true  anorthite.  Outside  the  core  basic  labradorite 
near  AbjAug  is  succeeded  by  a  third  zone  of  oligoclase  near  AbgAnj, 
and  outside  of  all  there  comes  a  narrow  zone  of  the  albite  Ab^gAn^.  The 
average  of  several  determinations  on  Carlsbad  albite  twins  gave  basic 
oligoclase,  Ab^Anj,  as  the  average  feldspar  of  the  rock.  The  usual 
accessories  are  present.  This  rock  is  a  typical  augite-biotite-diorite. 
A  very  similar  type  occurs  in  the  form  of  a  series  of  parallel  dikes 
on  the  northern  slope  of  Little  Ascutney  and  at  its  eastern  end  (spec. 
184).  Here  a  significant  amount  of  orthoclase  was  discovered  among 
the  accessories.  In  immediate  association  with  the  first  group  of 
reticulate  dikes  mentioned,  another  group  of  a  lighter  color  but  of 
similar  structure  showed  in  the  microscopic  examination  a  still  greater 
proportion  of  orthoclase,  which  is  accompanied  by  microperthite.  The 
bulk  of  the  feldspar  is  still,  however,  near  the  basic  oligoclase  AbgAn^. 
Biotite  is  the  only  other  essential.  Augite  fails  and  brown  hornblende 
is  a  rare  accessory.  The  type  may  be  called  a  hiotite-diorite  bearing 
accessory  orthoclase  and  microperthite. 

"WINDSORITE"   DIKES   CUTTING  THE  BASIC  STOCK. 

Potash-feldspar  finally  becomes  of  nearly  equal  importance  among 
the  essential  minerals  of  these  rocks  in  a  set  of  light-colored,  pinkish- 
gray  dikes  1  to  3  feet  (0.3  m.  to  0.9  m.)  in  width;  traversing  the  stock  in 
the  notch  just  northeast  of  the  easterii  end  of  Little  Ascutney  (spec. 
77),  and  again  on  the  notch  road  near  Mr.  Pierson's  house.  The 
dikes  at  the  former  locality  seem  to  have  been  cut  off  by  the  jjorphyry 
occurring  on  Little  Ascutney.  The  plagioclase  varies  from  andesine, 
AbgAug,  to  oligoclase,  AbgAuj,  the  average  mixture  being  probably 
basic  oligoclase,  AbaAn^.  The  triclinic  feldspar  is  often  surrounded 
by  mantles  of  orthoclase  or  microperthite.  Orthoclase,  microperthite, 
and,  probably,  soda  orthoclase,  especially  the  first  two  named,  are  the 
alkaline  feldspars,  the  abundance  of  which  is  reflected  in  the  chemical 
analysis  of  this  rock  type  (Table  VI,  column  1).^'  Shreds,  irregular 
plates,  and,  rarely,  idiomorphic  crystals  of  biotite  represent  the  only 
other  essential.     Rare  grains  of  augite  and  still  rarer  bleached  indi- 

a  Through  a  mistake,  a  fragment  of  diorite  from  the  younger  dikes  was  included  in  the  sample 
of  this  rock  sent  to  Washington  for  analysis.  This  unfortunate  fact  explains  the  difference 
between  column  K  (the  vitiated  analysis)  and  column  L  on  page  69  of  Bulletin  148  and  on  page 
g5  of  Bulletin  168  of  this  Survey. 


46  GEOLOGY    OF    ASOUTNEY    MOUNTAIN,   VERMONT.         [bull. 209. 

viduals  of  hornblende  with  ilmenite,  apatite,  zircon,  and  (doubtfully) 
titanite,  compose  the  list  of  accessories.  Py rite  developed  secondarily 
on  the  joint  planes  explains  the  sulphide  of  the  analysis.  The  mica 
must  be  rich  in  magnesia  and  is  probably  a  meroxene.  Quartz  occurs 
interstitially  in  comparatively  large  amount.  The  bisilicate  and  biotite 
exhibit  a  great  amount  of  magmatic  resorption,  and  it  is  therefore 
difficult  to  be  certain  of  the  order  of  crystallization.  It  is  ]3robably 
as  follows : 

Apatite. 

Zircon. 

Ilmenite  and  titanite. 

Biotite,  aiigite,  and  hornblende 

Andesine  and  oligoclase. 

Micropertliite,  orthoclase,  and  soda  orthoclase. 

Qnartz. 

No  analysis  of  the  mica  has  been  made.  On  account  of  its  small 
amount  in  the  rock,  no  serious  error  in  the  calculation  of  the  other 
and  more  important  essentials  will  be  made  if  we  assume  that  in  the 
biotite  there  is  20  per  cent  MgO,  40  per  cent  SiOg,  and  8  per  cent  K2O. 
On  this  supposition,  the  quantitative  mineralogical  composition  of  the 
rock  was  calculated  as  follows: 

Mineralogical  composition  of  ivindsorite. 

Per  cent. 

Albite  molecule 38. 5 

Orthoclase  molecule  .  - 28. 5 

Anorthite  molecule 11.5 

Quartz 13.0 

Biotite 5.0 

Magnetite  and  ilmenite 2.5 

Diopside,  apatite,  and  zircon 1.0 

If  the  average  plagioclase  =  AbjAn^,  the  rock  contains  34.5  j)er  cent 
soda-lime  feldspar  and  44  per  cent  alkali  feldspar. 


DALY.]  DIKES    CUTTING    THE    BASIC    STOCK. 

Table  VI. — Analyses  of  windsorite  and  other  rocks. 


47 


1. 

2. 

3. 

4. 

.5. 

SiO,  

ALOo 

64.62 
16.46 
1.82 
2.14 
1.10 
2.39 
4.57 

67.14 
15.37 
2.24 
1.93 
1.36 
3.60 
3.29 
4.06 
0.59 
0.07 

65. 00 
16.00 
1.50 
3.00 
2.00 
5.00 
3.50 
2.25 

59. 00-68.  50 

14. 00-17. 00 

■       1.50-2.25 

65.65 

16.84 

Fe^O, .   _ 

1 

FeO              

!^          4  01 
1.50-  4.50    1 

MgO 

CaO 

Na^O 

1.00-  2.50              0.13 
3.00-  6,50  :             2.47 
2. 50-  4. 50  '            5. 04 

K2O 

5.21 
0.39 

1.00-  3.50  1             5.27 

H,0  above  110°  C         

1           0.30 

H,0  below  110°  C 

CO2 

0.13 
0.11 
0.81 
0.03 
0.21 
0.05 
0.19 
0.12 
0.03 

Trace. 

Trace. 

Trace. 

. 

TiOa 

ZrOj 

. 

P,0= 

CI 

FeS^ 

MnO 

BaO 

SrO 

Li^O  

CuO 

Remainder. 

0.35 

1.75 

100. 38 
0.01 

100.00 

100. 00 

99.71 

0— F,C1 

100. 37 
0.10 

Total  S 

Sp.gr 

2. 666 

1.  Dike  of  windsorite,  Little  Asciitney  Mountain;  analysis  by  Hillebrand. 

2.  Average  of  two  typical  quartz-monzonites,  from  the  Sierra  Nevada.  Tnrner, 
Jour.  Geol..  Vol.  VII,  1899,  p.  152. 

3.  Average  composition  of  granodiorite,  according  to  Lindgren.  Seventeenth 
Ann.  Rept.  U.  S.  Geol.  Survey,  Pt.  II,  1896,  p.  35. 

4.  Limits  of  variation  in  granodiorite,  according  to  Lindgren.     Ibid.,  p.  35. 

5.  Alkaline  augite-hornblende-syenite  (nordmarkite),  Diana,  New  York;  ana- 
lyzed by  C.  H.  Smyth,  Bull.  Geol.  Soc.  Am.,  Vol.  VI,  1895,  p.  274. 

Table  VI  represents  type  analyses  of  related  rocks.  Of  these  and 
of  the  Ascutney  dike  the  essential  mineralogical  composition  is  as 
follows: 

1.  Basic  oligoclase,  microperthite,  orthoclase,  quartz,  biotite. 

2.  Oligoclase,  quartz,  orthoclase,  biotite,  amphibole. 


48  GEOLOGY    OF    ASOUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 

3.  Oligoclase-andesine  (usually  andesine) ,  quartz,  orthoclase,  biotite,  green  horn- 
blende. 
5.  Microperthite,  albite,  augite,  hornblende. 

This  rock  belongs  to  another  type  intermediate  between  the  ortho- 
clase rocks  and  the  plagioclase  rocks.  It  is  almost  identical  in  chem- 
ical com]30sition  with  certain  nordmarkites  (cf.  col.  5),  but  the  lime 
is  practically  all  in  the  highly  important  essential,  basic  oligoclase. 
This  character  definitively  removes  the  rock  from  the  nordmarkites. 
We  can  not,  on  account  of  the  high  alkalies  and  relatively  low  lime, 
place  it  in  the  group  of  the  granodiorites  (cf.  cols.  3  and  4),  nor,  for 
the  same  reason,  in  the  group  of  the  quartz- monzonites  (cf.  col.  2), 
though  in  general  the  affinities  are  stronger  with  the  last-named  group 
than  with  any  other  already  well-defined  type.  The  soda  and  the 
combined  alkalies  are  too  high  to  characterize  a  normal  lime-alkali 
quartz-syenite.  The  rock  is,  in  reality,  a  leukocratic  analogue  of  the 
quartz-monzonites  in  which  augite  is  replaced  by  biotite.  It  may  also 
be  considered  as  the  alkaline  equivalent  of  granodiorite.  Standing  in 
a  class  by  itself,  both  with  respect  to  the  other  Ascutney  intrusives 
and  with  respect  to  the  types  now  recognized  in  our  rock  classifica- 
tions, the  name  windsorite  is  proposed  for  the  rock  in  order  to  fix  this 
type  and  to  facilitate  reference  to  it.  The  name  is  taken  from  that 
of  the  neighboring  town  northeast  of  the  main  mountain.  Windsor- 
ite may  be  defined  as  a  leukocratic,  hypidiomorphic-granular  rock, 
composed  essentially  of  alkaline  feldspar  (microperthite  and  ortho- 
clase), basic  oligoclase,  quartz,  and  biotite,  and  characterized  by  high 
alkalies  (potash  slightly  in  excess  of  the  soda),  relatively  low  lime 
(contained  essentially  in  the  plagioclase),  low  iron,  and  low  magnesia. 

SYENITES. 

The  Basic  stock  is  cut  by  several  large  independent  bodies  of  syenitic 
habit.  Their  rocks  are  so  similar  in  compositon  that,  as  in  the  case  of 
the  dioritic  rocks,  a  detailed  ]3etrographical  description  of  a  chief 
phase  in  one  of  the  bodies  will  suffice  to  illustrate  the  larger  part  of 
what  may  be  said  in  description  of  the  other  phases  and  related  intru- 
sives.    In  this  way  some  repetition  may  be  avoided. 

MAIN   SYENITE  STOCK;  ITS   PHASES. 

As  we  have  already  seen,  Mount  Ascutney  owes  its  strong  relief 
to  the  largest  intrusive  mass  in  the  area  discussed — the  Main  syenite 
stock  covering  about  4  square  miles  (10. 5  square  kilometers) .  The  intru- 
sive character  of  the  rock  is  plainly  indicated  at  almost  any  j)art  of  the 
contacts  with  the  diorites,  gneisses,  or  phyllites  (see  PL  VI).  The  walls 
of  the  conduit  appear  to  be  usually  nearly  vertical,  inasmuch  as  the  line 
of  contact  in  all  but  two  or  three  cases  runs  straight  across  the  radiat- 
ing gulches  and  does  not  turn  up  or  down  the  corresponding  brook 


DALY.j  MAIN    SYENITE    STOCK.  49 

beds.  The  latter  rule  is  departed  from  at  three  of  the  largest  ravines  in 
the  mouiitain.  At  Crystal  Cascade  the  schists  stand  vertical  or  dip  at 
high  angles  to  the  east-northeast.  They  form  a  blunt  projection  into 
the  igneous  body,  and,  on  account  of  their  relative  softness,  the  strong 
gulch  below  the  cascade  has  been  worn  out.  The  actual  surface  of 
contact  between  schist  and  syenite  is  exposed  for  a  vertical  distance  of 
100  feet.  That  sample  contact  is  nearly  vertical.  A  second  deep 
ravine,  1  mile  east  of  the  cascade,  may  be  explained  as  located  on 
a  similar  broad  tongue  of  schist  less  resistant  to  the  weather  than  the 
syenite  to  right  and  left  of  the  ravine.  At  only  one  point  does  the  sur- 
face of  contact  seem  to  depart  from  verticality,  namely,  at  the  pictur- 
esque ravine  south  of  Brownsville.  There  the  schists  dip  under  the 
syenite  as  if  the  latter  had,  during  intrusion,  followed  the  planes  of 
schistosity  after  the  manner  of  a  sill  or  laccolith.  But  this  observa- 
tion stands  alone,  and  such  a  structural  relation  must  be  regarded  as 
exceptional  and  very  local. 

The  syenite  showed  a  general  independence  of  the  structure  of  the 
invaded  rocks  as  it  found  its  way  up  from  its  deep-ljang  source.  Both 
to  north  and  to  south  of  the  mountain  the  schists  strike  steadily  toward 
the  stock.  They  are  not  essentially  displaced  from  their  original  tilted 
position,  except  as  a  result  of  some  relativelj^  slight  crumpling  in 
the  contact  zone,  but  are  cut  squarelj^  off  by  the  syenite.  This  is  true 
at  the  eastern  contact  as  well.  At  several  points  the  contact  plane 
was  observed  to  cut  across  the  structural  plane  of  the  phyllites.  If, 
however,  the  intrusion  had  been  controlled  hy  the  latter,  we  should 
expect  the  surface  of  contact  to  dip  eastward  and  the  zone  of  meta- 
morphic  change  in  the  schists  to  be  broader  there,  at  the  easteim  end, 
than  elsewhere.     The  facts  do  not  agree  with  either  conclusion. 

The  syenite  thus  constitutes  a  pipe-like  stock  of  roundish  outline, 
the  cylindrical  form  being  modified  by  a  few  large  projections  of 
schist  in  place  and  b}'^  the  irregular  stock  of  the  younger  Ascutney- 
ville  granite.  - 

A  notable  characteristic  of  the  Main  stock,  as  of  the  older  one, 
is  the  variability  of  the  rocks  composing  it.  Though  they  are  every- 
where related  to  the  group  of  the  alkaline  syenites,  thej'^  exhibit 
important  mineralogical,  chemical,  and  structural  differences.  Four 
chief  types  of  the  variations  in  color,  grain,  structure,  proportion  of 
dark-colored  to  light-colored  constituents,  and  the  distribution  of 
inclosed  basic  segregations,  are  to  be  distinguished.  In  the  field  the 
transition  of  these  types  into  one  another  is  so  complete  that  they 
must  be  regarded  as  the  differentiated  product  of  one  body  of  magma. 
As  yet  there  is  no  certain  observation  forthcoming  to  show  that  there 
was  more  than  one  eruptive  period  for  all  four,  even  in  the  sense  of 
the  intimately  associated  diorite  and  reticulate  dikes  of  the  Basic 
stock,  or  in  the  sense  of  Brogger's  hypothesis  of  the  cutting  of  still 
unconsolidated  augite-syenites  by  elseolite-syenite.  It  will  be  remem- 
Bull.  209—03 4 


50  GEOLOGY    OF    ASCUTNEY   MOUNTAIN,  VERMONT.         [bull, 209. 

bered  that  Brogger  introduced  that  hypothesis  in  order  to  explain  the 
field  association  of  the  Christiania  rocks.'* 

Like  the  still  younger  granite,  the  syenite  has,  in  general,  a  finer 
texture  than  the  gabbro-diorites.  This  contrast  is  to  be  related  to 
the  greater  basicity  of  the  latter  rather  than  to  any  essential  differ- 
ence of  physical  conditions  under  which  the  intrusion  of  the  basic 
and  acid  stocks  occurred. 

Phase  f,  nordmarkite  of  granitic  habit. — The  only  quarry  that  has 
recently  been  worked  in  the  Ascutney  area  is  situated  within  a  few 
hundred  feet  of  the  contact  with  the  schists  in  the  first  of  the  four 
phases  of  the  Main  syenite.  Various  attempts  have  been  made  to  use 
the  stone  for  monuments  and  for  ornamental  purposes  generally,  but, 
for  a  reason  which  will  be  noted  further  on,  a  market  could  not  be 
permanently  secured  by  the  owners.  The  quarry  seems  to  have  been 
practically  abandoned.  The  finest  blocks  yet  taken  out  are  doubtless 
those  which  are  to  be  seen  in  the  large  columns  of  the  library  build- 
ing at  Columbia  University,  New  York  City. 

The  rock,  as  represented  in  the  quarr}^,  is  a  handsome,  dark-green 
sj'enite,  in  this  iDlace  characterized  hy  medium  to  coarse  grain  and  a 
typical  eugranitic  structure  (spec.  42) ;  elsewhere  this  i^hase  grades 
into  one  possessing  a  trachytic  structure.  It  is  a  syenite  with  variable 
amounts  of  free  quartz  and  a  low  percentage  of  colored  constituents. 
Primary  veins  or  flow  streaks  are  common;  they  are  usually  finer- 
grained  than  the  average  rock,  and  are  even  more  poorly  ]3rovided 
with  bisilicates.  In  addition  to  the  feldspars  and  accessory  quartz, 
the  list  of  minerals  includes,  in  the  order  of  their  abundance,  a  horn- 
blende, biotite,  a  pyroxene,  allanite,  titaniferous  magnetite,  apatite, 
pyrite,  zircon,  monazite,  and  a  lime-iron  garnet.  The  order  of  their 
crystallization  seems  to  have  been  as  follows: 

Apatite. 

Zircon. 

Magnetite,  pyrite,  garnet. 

Monazite  and  allanite. 

Augite,  hornblende,  and  biotite. 

Oligoclase. 

Alkaline  feldspars. 

Quartz. 

The  feldspars. — The  constituents  which  determine  the  structure, 
texture,  and  color  of  the  syenite  are  the  feldspars  (PI.  IV,  A.)  Of 
these,  microperthite  is  by  far  the  most  abundant,  and  with  it  are 
associated  orthoclase,  soda-orthoclase,  microcline,  and  a  plagioclase. 
There  is  no  observable  difference  in  the  macroscopic  habit  of  these 
feldspars,  and  it  was  only  by  the  careful  study  of  slides  and  rock 
powder  that  all  the  species  could  be  determined.  All  of  them  are 
undoubtedly  the  product  of  primary  crystallization. 

oZeit.  fur  Kryst.,  Vol.  XVI,  1890,  p.  281. 


DALY.]  MAIN    SYEmTE    STOCK.  51 

The  microperthite  is  especially  interesting  on  account  of  its  typical 
development.  The  usual  intergrowth  is  that  of  orthoclase  with  a 
plagioclase  varjang  from  albite  to  an  acid  oligoclase  near  AbgAn^, 
but  on  many  cleavage  pieces  it  was  easily  proved  that  the  triclinic 
feldspar  was  intergrown  with  a  monoclinic,  itself  strongly  charged 
with  the  soda  molecule.  Such  individuals  gave  extinctions  of  12°  and 
19°  on  (010)  for  the  two  kinds  of  lamellae,  thus  indicating  the  associa- 
tion of  nearly  pure  albite  with  an  orthoclase  that  stands  at  the  extreme 
soda  end  of  the  series,  generally  designated  by  the  name  "soda- 
orthoclase."  In  all  cases  the  intergrowth  follows  the  law  whereby 
the  triclinic  lamellae  lie  in  the  monoclinic  feldspar  parallel  to  a  steep 
orthodome;  the  angle  of  72°  between  the  albite  lamellse  and  the  basal 
cleavage  on  (010)  indicates  that  this  dome  may  be  (801),  the  one  noted 
in  this  relation  to  intergrowth  by  Brogger.  From  the  normal  micro- 
perthite there  are  all  transitions  to  what  would  appear  to  be  true 
cryptoperthite.  Both  ends  of  the  series  sometimes  show  the  mui-chi- 
sonite  parting,  which  is  unusually  clean  and  definite. 

The  tabular  crystals  of  well-lamellated  microperthite  from  a  highly 
feldspathic  phase  on  the  Brownsville  slope  of  the  mountain  represent 
a  very  high  proportion  of  soda  in  the  mixture,  as  shown  by  the  polar- 
ization phenomena  and  the  specific  gravity  of  from  2.610  to  2.611  at 
17°  C.  Generally,  however,  the  proportion  of  potash  to  soda  is  about 
1:1,  corresponding  to  a  specific  gravity  of  from  2.584  to  2.595  at  the 
same  temperature.  The  same  average  ratio  is  believed  to  character- 
ize the  feldspars  of:  the  rock  as  a  whole.  It  is  true  that  there  is  a  not 
unimportant  amount  of  orthoclase  and  soda-orthoclase  in  most  of  the 
slides,  yet  this  lowering  of  the  otherwise  high  percentage  of  soda  is 
occasionally  counterbalanced  by  a  little  free  oligoclase  and  always  by 
a  microperthite  which  is  richer  in  the  triclinic  component  than  the 
average  stated. 

The  pure  potash  feldspar  is  relativelj^  rare.  It  occurs  as  orthoclase 
and  as  microcline,  both  contemporaneous_with  the  microperthite  in 
their  period  of  crystallization. 

The  plagioclase  is  no  more  than  accessory.  The  usual  optical 
methods  of  determination  agreed  in  showing  that  it  belongs  to  a  series 
from  practically  pure  albite  to  the  oligoclase  AbgAuj.  An  orthoclase 
could  not  be  demonstrated  in  any  of  the  Ascutney  rocks,  nor  should 
it,  on  account  of  the  lowness  of  lime,  be  expected.  Barium  oxide 
doubtless  occurs  in  isomorphic  relation  with  the  soda  and  potash  of 
the  feldspars.     No  hyalophane  has  been  discovered  in  the  rock. 

Rapid  tarnishing  on  exposure  to  air. — One  of  the  most  remarkable 
properties  of  this  rock  consists  of  the  unstable  character  of  its  color. 
When  broken  out  of  the  quarry  a  fresh  specimen  is  uniformly,  on  the 
surface  of  fracture,  a  light  bluish  gray.  In  the  course  of  twenty-four 
hours,  under  atmospheric  conditions,  this  tint  changes  to  one  with  a 
greenish  tinge,  and  after  an  exposure  to  the  air  of  ab®ut  thirty  days 


52  GEOLOGY    OF    ASCUTISTEY   MOUNTAIN,   VERMONT.         [bull. 209. 

it  has  become  a  deep  brownish  green — the  color  we  have  noted  for  the 
numerous  blocks  of  the  quarry.  This  green  color  is  in  its  turn  lost 
when  the  rock  has  suffered  more  pronounced  weathering  after  many 
years  of  exposure.  The  final  change  gives  the  familiar  yellows  and 
browns  of  a  decomposed  ferruginous  rock.  In  this  stock  the  rapid 
change  from  gray  to  green  was  observed  only  in  phase  /,  as  exposed 
on  the  north  and  northwest  slopes  of  the  mountain. 

Examination  quickly  showed  that  the  color  change  of  the  rock  is 
conditioned  by  the  feldspar  and  that  it  is  altogether  a  superficial 
phenomenon,  taking  place  only  where  the  air  has  access  to  the  min- 
eral. The  question  has  naturally  arisen  as  to  the  cause  of  this  pecul- 
iar instability  of  color,  and  a  number  of  experiments  were  carried 
out  which  have  thrown  light  on  the  problem.  To  show  that  one  or 
more  of  the  principal  atmospheric  gases  were  essential  to  the  reac- 
tion, a  gray  piece  of  the  fresh  rock  was  immersed  in  a  stream  of 
carbon  dioxide  gas  for  twenty  minutes  and  then  kept  in  an  atmosphere 
of  that  gas  for  twenty-four  hours.  No  appreciable  change  was  noted 
in  the  original  gray  tint,  showing  that  in  all  probability  the  carbonati- 
zation  of  some  unknown  element  in  the  feldspar  could  not  explain 
the  alteration  of  tint.  The  inference  was  ready  to  hand  that  it  was 
rather  due  to  oxidation.  A  gray  fragment  of  the  rock  was  accord- 
ingly placed  in  an  atmosphere  of  purified  oxygen  over  niglit.  A  per- 
ceptible change  to  the  green  color  resulted.  The  same  piece  was 
then  changed  to  -an  intense  green  by  an  exposure  of  thirty  minutes  to 
a  stream  of  oxygen,  while  the  fragment  was  kept  at  a  temperature  of 
about  150°  C. 

The  further  question  remained:  What  oxidizable  substance  pres- 
ent in  the  feldspar  would,  on  uniting  with  the  oxygen  of  the  air, 
furnish  the  required  color?  That  it  is  not  organic  was  shown  by 
the  fact  that  before  the  blowpipe  the  green  tint  was  not  only 
not  destroyed,  but,  on  the  contrary,  was  deepened  in  the  oxidizing 
flame — another  testimony  to  the  fact  of  oxidation  as  the  true  cause. 
Partial  decolorization  resulted  from  the  application  of  the  reducing 
flame.  The  probable  explanation  of  the  color  change  is  found  in 
the  oxidation  of  the  ferrous  oxide  of  the  feldspars  to  the  ferric, 
thus  giving  a  yellow  which,  in  combination  with  the  fundamental 
blue-gray  of  the  under  layers  of  the  crystal  substance,  affords  the 
green  of  the  altered  surface.  In  acids  the  mineral  is  decolorized  to 
the  original  bluish  gray,  which  is  permanent,  and  the  filtrate  gives  a 
strong  reaction  for  iron.  A  iiigh  power  of  the  microscope  shows  that 
the  perfectly  fresh  feldspars  are  all  crowded  with  myriads  of  extremely 
minute  blackish  granules.  It  is  possible  that  this  dust  is  composed 
of  ferrous  oxide,  dating  as  to  its  period  of  formation  from  the  time  of 
the  original  crystallization  of  the  rock.  If  this  be  true,  we  can  derive 
the  instability  of  color  from  one  of  the  most  familiar  reactions  in  the 
history  of  metasomatic  processes. 

Were  the  coloring  substance  uniformly  distributed  throughout  the 


DALY.]  MAIN    SYENITE    STOCK.  53 

body  of  the  rock,  this  syenite  would  make  a  favorite  material  for 
decorative  purposes,  for  it  is  capable  of-  a  fine  polish;  but  the  distri- 
bution is,  unfortunately,  very  uneven,  and  the  consequence  is  that  the 
polished  monument  or  shaft  is  often  blemished  with  streaks  of  lighter 
and  darker  hue  than  the  average.  Furthermore,  as  already  implied, 
the  tint  of  any  specimen  can  never  be  said  to  be  permanent.  As  the 
oxidation  progresses,  the  bluish  tone  of  the  feldspar  substance 
beneath  the  surface  will  have  less  and  less  influence  on  the  color 
mixture  and  a  more  brownish  tone  will  result.  This  is  what  has  actu- 
ally happened  in  the  case  of  several  tombstones  which  have  stood  for 
some  years  in  the  cemeteries  of  Brownsville  and  Windsor. 

A  similarly  rapid  change  of  color — from  a  grayish  green  to  a  more 
pronounced  green — on  exposure  to  the  air,  has  been  described  by 
Cushing  as  characterizing  the  squeezed  augite-syenites  near  Loon 
Lake,  New  York."  He  suggests  staining  from  the  oxidation  of  the 
ferrous  iron  derived  from  decomposed  hypersthene  as  a  possible  cause 
of  the  rusty  brown  color,  but  leaves  open  the  question  of  the  causes 
of  the  early  stages  of  the  color  change.  He  points  out  that  the 
uncrushed  crystals  are  alwaj^s  less  green  than  the  feldspar  granulated 
by  pressure.  This  would  be  expected  as  one  result  of  the  increased 
ease  with  which  oxidizing  fluids  would  circulate  in  the  rock  after 
crushing.  The  gray  color  of  the  Ascutney  rock  corresponds  with  its 
other  properties  in  showing  that  it  has  not  been  subjected  to  such 
squeezing  as  that  once  suffered  by  the  New  York  syenite,  which  in  other 
respects  is  strikingly  similar  to  our  rock.  Types  very  close  to  botli 
of  these  in  nature  and  origin  occur  at  Killington  Peak  in  western  Ver- 
mont and  at  Shefford  Mountain,  Quebec,  and  possess  the  same  pecu- 
liar green  color.  At  the  latter  locality  the  change  from  the  fresh  gray 
to  green  has  also  been  observed.  It  may  be  noted  in  passing  that 
green  is  a  favorite  hue  for  several  species  of  alkaline  rocks.  Tin- 
guaites  are  commonly  green,  like  the  groundmass  of  pantellerites, 
and  grorudite  from  the  classic  locality  is  green,  the  color  in  the  last 
mentioned  rock  being  due,  however,  to  the  essential  tegirine. 

Hornblende. — The  next  most  important  constituent  of  the  sj^enite  is 
a  hornblende  belonging  to  the  alkali-iron  group  of  amphiboles.  Often 
idiomorphic  against  the  feldspars,  it  yet  commonly  possesses  the  fea- 
ture characteristic  of  hornblendes  that  have  grown  in  an  alkaline 
magma — namely  the  irregular  outline  due  to  resorption.*  Within  the 
cavities  thus  formed  by  this  magmatic  solution,  feldspar  and  quartz 
have  crystallized,  and  in  section  have  the  appearance  of  inclusions  in 
the  hornblende.  The  color  of  the  mineral  varies  through  shades  of 
brown  according  to  the  following  scheme : 

a,  light  greenish  brown  to  grayish  yellow. 

b,  deep  greenish  brown  to  olive-brown. 

c,  grayish  olive-green. 
r- 


o^ 


aBvLll.  Geol.  Soc.  Am.,  Vol.  X,  1899,  p.  178. 

bCt.  Brogger,  Zeitschr.  fiir  Kryst.,  Vol.  XVl,  p.  131. 


54  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.  [bull.209. 

Tlie  absorption  parallel  to  b  and  c  is  very  strong;  b>c>a. 

By  the  use  of  cleavage  pieces  mounted  on  the  Fedorotf  table,  the 
extinction  was  determined  on  (010)  at  16°.  The  extinction  on  the  cleav- 
age plate  itself  was  found  to  average  14°  39'.  By  turning  the  plate 
about  an  axis  at  once  coincident  with  the  vertical  axis  of  the  crystal 
and  parallel  with  the  principal  section  of  the  polarizer,  it  was  possible  to 
test  the  curve  of  extinction  in  the  vertical  zone.  Readings  were  taken 
at  positions  of  the  cleavage  plate  where  the  plane  of  symmetry  made 
angles  of  42°  30',  47°  30',  and  77°  30'  with  the  plane  passing  through 
the  crystal  at  right  angles  to  the  axis  of  the  microscope.  The  corre- 
sponding angles  of  extinction  were  found  to  be  16°  50',  17°  25',  and 
12°  0'.  These  results  mean  an  angle  of  extinction  on  (010)  of  16°,  and 
an  optical  angle  of  about  70°  for  the  amphibole.**  Etch  figures  on  a 
cleavage  plate  immediately  oriented  the  crystal  and  therewith  the 
ellipsoid  of  optical  elasticity.*  The  optical  axis  c  lies  in  the  obtuse 
angle  /i  in  Tschermak's  orientation.  These  conclusions  were  checked 
by  the  close  study  of  rock  slides,  and  chance  sections  of  the  hornblende 
favorable  to  the  rough  measurement  of  the  optical  angle  and  to  the 
determination  of  c  :  c  confirmed  the  results  derived  from  the  use  of 
cleavage  pieces. 

The  usual  twinning  parallel  to  (100)  was  observed. 

The  angle  of  the  cleavage  prism  was  measured  on  about  twenty 
individuals  and  found  to  vary  from  55°  ]3'  to  56°  0',  with  an  average 
of  55°  32'.  This  great  variation  from  the  mean  is  not  to  be  explained 
by  poor  reflexes  or  by  the  personal  equation  of  the  observer,  but  must 
be  conditioned  by  some  as  yet  unknown  cause  or  causes. ''  The  specific 
gravity  was  taken  with  the  Klein  solution;  it  averaged  3.272  at  17°C., 
varying  in  a  suite  of  ten  specimens  from  3.266  to  3.278.  A  thin  splin- 
ter of  the  mineral  melts  quietly  in  the  Bunsen  burner  with  a  strong 
soda  flame.  In  view  of  such  ];)roperties  we  can  place  this  hornblende 
near  barkevikite,  in  the  alkali-iron  series  developed  by  Brogger.*^ 
Poikilitic  intergrowths  with  biotite  and  allanite  and  parallel  inter- 
growth  with  augite  make  it  impossible  to  separate  the  hornblende  and 
thus  permit  a  chemical  anal^-sis  of  it  being  made,  but  it  is  plainly  rich 
in  ferric  oxide  and  soda. 

Augite. — Compared  with  the  amphibole,  the  pyroxene  is  present  in 
very  subordinate  amount.  It  almost  invariably  occurs  in  the  cores  of 
parallel  intergrowths  with  the  hornblende,  which  there  is  every  rea- 
son to  believe  is  j)rimary  and  has  thus  not  been  derived  from  the 
pyroxene  either  b}^  magmatic  or  by  metasomatic  changes.  The  augite 
has  the  usual  diopsidic  habit  of  most  augite-syenites;  the  optical  angle 


(iProc.  Am.  Acad.  Arts  and  Sciences,  Vol.  XXXIV,  1899,  p.  311. 

b Ibid.,  p.  373. 

c  There  is  need  for  a  thorough  investigation  of  the  whole  amphibole  group  for  the  purpose  of 
fixing  the  series  of  prismatic  angles,  as  they  undoubtedly  vary  with  the  chemical  com.position, 
and  the  student  of  the  amphiboles  would  probably  be  repaid  if  he  set  about  the  task  of  finding 
the  possible  causes  for  the  noteworthy  variation  in  the  angle  for  the  same  species  from  one 
locality. 

f'  Ge.steine  der  Groi-udit-Tinguait  Serie,  p.  SS. 


BALY.]  MAIN    SYENITE    STOCK.  55 

is,  however,  remarkably  small,  45°  15'  being  measured  in  oil.  The 
extinction  angle  was  not  found  on  account  of  the  lack  of  favorable 
material. 

Biotite. — In  about  equal  proportion  with  the  pyroxene  is  a  deep- 
brown  primary  mica  characterized  by  normal  properties.  The  optical 
plane  and  the  plane  of  symmetry  are  coincident.  From  the  low  per- 
centage of  magnesia  in  the  total  analj^sis  it  appears  that  the  mica  is  a 
true  lepidomelane  and  not  a  meroxene.  The  formation  of  skeleton 
crystals  by  magmatic  resorption  is  here  also  very  striking. 

Allanite. — The  rock  of  the  Windsor  quarry  contains  an  important 
accessory  not  recognized  in  any  other  part  of  the  syenite  stock.  It 
occurs  in  the  form  of  elongated  anhedral  grains,  either  independent 
or  associated  as  irregular  intergrowths  with  the  hornblende.  In  the 
hand  specimen  the  mineral  can  be  readily  made  out  by  its  black 
color,  waxy  to  lustrous  apjjearance,  and  by  the  presence  of  only  one 
good  cleavage.  In  many  cases  the  individuals  are  as  much  as  half  a 
centimeter  long.  The  most  striking  microscopic  property  is  the 
extremely  strong  pleochroism  and  absorption.  The  colors  varj^  from 
cinnamon -brown  to  deep  walnut-brown  in  some  individuals;  in  others 
chestnut-brown  and  purplish  brown  appeared,  while  in  the  thicker 
slides  the  more  powerful  absorption  gave  almost  absolute  black- 
ness. The  single  refraction  seemed  to  be  higher  than  that  of  horn- 
blende, but  the  double  refraction  was  weaker.  In  addition  to  the  good 
cleavage  visible  macroscopically,  there  was  also  present  a  less  jjerfect 
cleavage  transverse  to  the  former  at  a  high  angle.  Before  the  blow- 
pipe the  cleavage  pieces  fused  with  intumescence  to  a  black  magnetic 
glass.  Such  an  association  of  pro^Derties  seemed  to  indicate  allanite, 
and  an  examination  of  some  material  from  Suhl  (orthite)  confirmed 
the  close  similarity  with  that  mineral.  The  conviction  became  a  prac- 
tical certainty  when  some  fragments  were  dissolved  in  hydrofluoric 
acid,  and  from  the  solution  an  excellent  test  for  cerium  was  obtained 
by  precipitating  with  ammonium  oxalate. 

The  allanite  is  (on  account  of  the  strong  magmatic  resorption)  never 
idiomorphic.  Yet  it  must  be  one  of  the  oldest  constituents  of  the  rock, 
as  it  is  inclosed  by  the  hornblende,  in  which  it  often  forms  lively 
pleochroic  halos.  The  two  minerals  are  sometimes  intergrown,  but 
the  allanite  never  incloses  the  other.  Apatite,  zircon,  and  magnetite 
antedate  both  in  the  order  of  crystallization. 

We  have  here,  then,  one  more  example  showing  the  importance  of 
allanite  in  eruptive  rocks.  As  early  as  1885  Iddings  and  Cross  noted 
the  occurrence  of  the  mineral  at  28  localities  and  in  9  rock-types, 
including  granite,  gneiss,  granite-porphyry,  quartz-porphyry,  diorite, 
dacite,  and  rhyolite.^  Since  then  it  has  been  discovered  at  many  other 
localities,  including  some  where  the  rock  is  alkaline  and  related  in 
character  to  the  Ascutney  syenite,  e.  g.,  the  hornblende  granite  of 

aAm.  Jour.  Sci.,  3d  series,  Vol.  XXX,  1885,  p.  108. 


56  GEOLOGY    OF    ASCUTISTEY    MOUNTAIN,   VERMONT.         [bull. 209. 

Essex  County,  Mass./*  and  tlie  quartz-syenite  of  Loon  Lake,  New 
York.^ 

Monazite. — A.  second  accessory  which  was  attended  with  consider- 
able difficulty  in  its  determination  occurs  in  some  amount  in  the  quarry 
rock.  It  has  never  been  observed  macroscopically,  but  only  in  the 
slide,  where  it  is  found  in  the  form  of  roundish  grains  reaching- 1  milli- 
meter in  diameter.  These  are  nearly  colorless,  with  a  grayish-yellow 
tint,  and  are  characterized  by  high  single  refraction  and  by  high  double 
refraction,  giving  polarization  colors  of  the  third  order.  Crystal  form 
is  always  lacking,  but  optical  tests  showed  the  mineral  to  be  biaxial 
and  monoclinic  or  triclinic.  The  cleavages,  about  at  right  angles  to 
each  other,  were  seen  in  the  section  of  one  small  individual.  The  min- 
eral was  found  to  be  difficultly  soluble  in  nitric  acid  and  more  easily 
in  hydrochloric  acid.  From  the  solution  a  precipitate  with  molybdate 
of  ammonia  was  obtained,  one  too  abundant  to  be  explained  by  the 
associated  apatite,  and  thus  showed  the  grains  to  belong,  without 
doubt,  to  a  i)hosphate.  The  quantity  of  the  solution  was  so  small  as 
to  render  impossible  the  sure  determination  of  the  rare  earths  which 
should  be  expected  if  the  mineral  be  really  monazite.  Yet  it  may  best 
be  ascribed  to  that  species  as  the  phosphate  nearest  in  optical  proper- 
ties to  the  one  with  which  we  are  dealing. 

The  grains  inclose  numerous  apatite  needles  of  great  minuteness  and 
a  few  square  sections  of  magnetite.  All  three  minerals  seem  to  have 
crystallized 'before  the  essential  constituents.  The  monazite  furtl^r 
shows  an  interesting  paragenesis  with  the  allanite,  the  latter  sometimes 
appearing  as  a  mantle  about  the  former.  Such  an  intimate  associa- 
tion of  a  phosphate  with  a  member  of  the  epidote  f  amilj^  is  rather  sur- 
prising, but  from  the  study  of  the  material  in  hand  both  minerals 
seem  to  be  primary. 

The  magnetite  is  titaniferous.  It  is  inclosed  as  a  primary  mineral 
by  all  the  other  constituents  except  apatite.  Its  habit  is  the  usual 
one  of  granitic  eruptives. 

Titanite  is  rather  less  common  than  in  the  diorites,  but  possesses 
the  same  features  as  in  the  older  rock.  It  incloses  apatite;  its  rela- 
tion to  zircon  is  indeterminable  as  to  the  period  of  crj^stallization. 

Apatite  is,  as  usual,  most  abundant  in  the  vicinity  of  the  bisilicates, 
and  is  accordingly  here,  as  in  the  f  eldspathic  phases  of  the  stock  as  a 
whole,  very  rare. 

Zircon  is  more  common  than  in  the  phases  of  the  Basic  stock.  Its 
habit  is,  however,  the  same,  excepting  that  it  here  shows  a  pronounced 
color  and  pleochroism. 

E,  pigment  irregularly  distributed— pale  violet  and  colorless. 

0,  solid  color — paler  violet. 

The  zircon  is  younger  than  the  apatite  and  seems  to  have  accom- 
panied  the  titaniferous  magetite   in   its    crystallization.     Irregular 


« Jour.  Geol.,  Vol.  VI,  1898,  p.  792. 

&H.  P.  Gushing,  Bull.  Geol.  Soc.  Am.,  Vol.  X,  1899,  p.  180. 


DALY.]  MAIK    SYENITE    STOCK.  57 

roundish  inclusions  with  wide  margins  of  total  reflection  are  ascribed 
to  imprisoned  gas. 

Quartz  is  uniformlj^  allotriomorphic  and  interstitial.  Fluid  cavities 
and  negative  crj^stals  are  very  numerous.  The  filling  material  of  the 
latter  could  be  well  studied  here  on  account  of  the  remarkable  per- 
fection of  the  forms.  In  many  cases  double  bubbles,  that  unite  on 
heating  the  preparation,  indicate  carbonic  acid  gas  in  a  saturated 
solution  of  water.  The  usual  orientation  of  the  negative  crystals  with 
their  chief  axes  parallel  to  that  of  their  host  is  easily  demonstrable, 
especially  in  the  isotropic  sections  of  the  quartz;  in  them  the  fluids 
lie  in  six-sided  cavities,  whose  sharp  outlines  are  of  exceptionally 
clear  definition. 

The  extremely  few  grains  of  reddish  common  garnet  were  found  in 
this  phase  only  in  those  thin  sections  made  from  specimens  collected 
near  the  schist  contact  and  are  doubtless  to  be  referred  to  slight 
endomorphic  influence  exercised  bj^  the  country  rock  on  the  eruptive. 

Basic  nodules,  from  1  to  2  inches  (2.(3  to  5.2  centimeters)  in  diameter 
are  occasionally  seen  in  the  quarry  rock.  They  are  differentiated  min- 
eralogically  from  their  parent  rock  simply  by  a  greater  richness  in 
hornblende,  which  is  here,  too,  strikingly  corroded.  One  can  not  be 
sure  that  the  poikilitic  habit  of  the  mineral  is  anything  more  than 
apparent;  primarj^  inclusion  of  quartz  and  feldspar  might  give  the 
same  appearance  in  thin  section  as  that  due  to  extensive  embaying  of 
the  hornblende  by  the  caustic  feldspathic  magma. 


PLATE   IV. 

A.  Typical  thin  section  of  the  nordmarkite  of  the  Main  stock,  granitic  phase, 
composed  ahnost  entirely  of  microperthite  and  qixartz;  crossed  nicols,  X  30.  (See 
p.  50.) 

B.  Pyroclastic  feldspar  (soda-orthoclase)  surrounded  by  a  reaction  rim  rich  in 
alkaline  hornblende,  from  the  large  paisanite  dike  on  northwest  slope  of  Ascutney 
Mountain;  crossed  nicols,  X  7.     (See  p.  71.) 

58 


U.  S.   GEOLOGICAL  SURVEY 


BULLETIN  NO.  209,  PL.  IV 


(B) 


HE    MERIDEN    GRAVURE    CO. 


DALY.]  MAIN    SYENITE    STOCK. 

Table  YII.— Analyses  of  nordmarkite  and  other  rocks. 


59 


1. 

2. 

3. 

4. 

5. 

60.03 

20.76 

j  4.01 

1   0.75 

0.80 

2.62 

5.96 

5.48 

a  0.  59 

6. 

7. 

8. 

SiOa -- 

65.43 
16.11 
1,15 
2.85 
0.40 
1.49 
5.00 
5.97 
0.39 
0.19 
Trace? 
0.50 
0.11 
0.13 
None. 
0.05 
0.08 
0.07 
0.03 
0.23 
Trace. 

64.88 

16.24 
1.37 
2.70 
0.89 
1.92 
5.00 
5.61 
0.46 
0.19 

None. 
0.69 
0.13 
0.13 

None. 
0.04 
0.08 

64.04 
17.92 
0.96 
2.08 
0.59 
1.00 
6.67 
6.08 

i  1.18 

60.45 
20.14 

[  3.  80 

1.27 
1.68 
7.23 
5.12 

0.71 

60. 5-67. 0 
20. 0-17. 5 

U.  0-  3.0 

1.0-  0.5 
2.0-  1.5 
7.0-  6.5 
5.0-  6.0 

61.49 
16.14 

5.81 

0.99 
1.67 
6.19 
5.70 

«1.17 

65.43 

16.96 

(  1.55 

1  1.53 

0  22 

Al.Oa 

Fe-A 

FeO 

MgO- 

CaO 

1  36 

Na,0 

5  95 

K^O- 

H.,0  above  110°  C. 

5.36 

0.82 

HjO  below  110°  C_- 
COo 

None. 

TiOa 

1  0.62 

0.16 

ZrO^ 

P2O, 

0.07 

0.53 

0.02 

SO3 

0.06 

CI 

0.04 

F 

FeSa 

BaO 

0.06 
0.14 

Faint 

trace. 

Trace. 

None. 

MnO 

0.23 

Trace. 

0.28 

0.40 

SrO 

LijO                           Stronsr 

trace. 

100. 18 
0.04 

100. 53 
0.04 

101.37 

100.40 

100. 07 

99.97 

99.86 

0— F  CI 

100. 14 
0.036 
2.659 

100. 49 

Totals 

- 

Sp.gr 

2.683 

a  Loss  on  ignition. 

1.  Hornblende-biotite-nordmarkite  of  granitic  structure,  Ascutney  Mountain 
(phase  /) ;  analysis  by  Hillebrand. 

2.  Hornblende-biotite-augite-nordmarkite  of  porphyritic  structure,  Ascutney 
Mountain  (phase  g) ;  analysis  by  Hillebrand. 

3.  Classic  nordmarkite,  Tonsenas,  Norway:  Brogger,  Zeitschr.  fiir  Kryst.,  Vol. 
XVI,  1890,  p.  54. 

4.  Classic  nordmarkite,  Auerod,  Norway:  Brogger,  ibid.,  p.  54. 

5.  Classic  pulaskite,  Fourche  Mountain:  Williams,  Arkansas  Geol.  Surv. ,  Ann. 
Rept.  for  1890,  Vol.  II,  p.  70. 

6.  Limits  of  variation  in  nordmarkites  and  related  quartz-syenites,  according 
to  Brogger,  op.  cit..  p.  81. 

7.  Average  analysis  of  three  syenite-porphyry  dikes  from  the  northern  Adiron- 
dacks;  Cushing.  Bull.  Geol.  Soc.  Am.,  Vol.  IX,  1898,  p.  248. 

8.  Nordmarkite  of  Shefford  Mountain;  Dresser,  Am,  Geol.,  Vol.  XXVIII,  1901, 
p.  209. 


60 


GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT. 


[BULL.  209. 


From  the  analysis  of  the  fresh  quarry  rock  (Table  VII,  col.  1)  the 
table  of  molecular  proportions  was  calculated  as  follows : 


Anal- 
ysis. 

Molecular 
proportions. 

SiOs                           -■--. 

65.43 
16.11 
1.15 
2.85 
0.23 
0.40 
1.49 
5.00 
5.97 
0.50 
0.11 
0.13 
0.05 

1.0905 
0. 1579 
0. 0079 
0. 0396 
0. 0032 
0.0100 
0. 0266 
0. 0806 
0. 0635 
0.0061 
0.0011 
0. 0009 
0. 0014 

AI2O3                           --   ---   

FeaOg                                 --         --   -     ---... 

FeO -- 

MnO 

MgO                         - 

CaO                                 

NaaO                                   -     - 

K.,0 

TiO.,                     

ZrO.,                       - - 

P.,0,                - 

CI 

A  partial  determination  shows  that  zircon  forms  0.2  per  cent  of  the 
rock;  apatite,  0.4  per  cent;  magnetite  (crediting  it  with  all  the  FegOg), 
1.8  per  cent.  A  careful  mechanical  separation  of  the  hornblende  per- 
mitted a  rough  estimation  of  its  total  amount;  slightly  impure  from 
included  and  intermixed  allanite,  biotite,  and  magnetite,  it  composed 
5.2  per  cent  by  weight  of  the  total  powder.  Allowing  for  its  impurity, 
we  shall  not  be  far  from  the  truth  in  regarding  5  per  cent  as  the  pro- 
portion of  hornblende.  Arbitrarily  estimating  the  lime  content  of 
the  hornblende  as  10  per  cent  (near  barkevikite),  the  proportion  of 
the  anorthite  molecule,  after  allowing  also  for  the  lime  in  the  apatite, 
was  calculated  at  4  per  cent.  On  the  supposition  that  all  the  soda 
occurs  in  the  albite  molecule  and  all  the  potash  in  the  ortlioclase 
molecule,  they  would  respectively  compose  42  and  35.3  per  cent  of 
the  rock.  Both  these  figures  must  be  slightly  too  high.  The  result 
of  the  whole  calculation  shows  the  following  approximate  composition: 

Per  cent. 

Albite  molecule 41. 0 

Orthoclase  molecule 35.  0 

Quartz 11.0 

Hornblende -■ -  5.0 

Anorthite  molecule 4.0 

Magnetite . 1-8 

Apatite 0.4 

Zircon J 0.2 

Biotite,  titanite,  diopside,  and  allanite 1.6 

100.0 
Three  determinations  of  the  specific  gravity  of  the  rock  gave  an 

average  of  2.659  at  17°  C. 


DALY.]  MAIN    SYENITE    STOCK.  61 

The  geological  relations,  structure,  and  constitution  of  this  phase 
clearly  place  the  rock  among  the  alkaline  quartz-syenites,  closelj^ 
allied  to  the  nordmarkites  of  TonsenS^s  and  other  localities  in  the 
Christiania  region  (compare  cols.  1,  3,  and  6).  Brogger's  table  gives 
the  limiting  values  in  the  percentage  composition  of  nordmarkite. 
Two  other  American  examples  are  noted  in  columns  7  and  8. 

Phase  g. — The  porphyritic  phase  of  the  stock  is  widespread,  espe- 
cially on  the  east  and  southeast  sides  of  the  mountain.  The  rock  is 
structurally,  but  neither  chemically  nor  mineralogically,  except  as 
regards  some  of  the  accessories,  to  be  distinguished  from  the  normal 
equigranular  type.  This  second  phase  is  exhibited  on  a  large  scale 
on  the  prominent  bald  knob  east  of  the  main  summit. 

The  color  of  the  rock  is  always  a  light  gray  or  pinkish  gray,  which 
is  stable  and  does  not  change  to  green  on  exposure  (spec.  115).  The 
phenocrysts  are  almost  always  roundish  feldspars  which  may  reach 
the  diameter  of  one  centimeter  or  more;  much  more  rarely  a  horn- 
blende or  augite  individual  will  approach  the  same  dimension.  The 
phenocrystic  feldspars  are  microperthite,  orthoclase,  albite,  and  oligo- 
clase,  often  arranged  in  groups  of  two  or  more  large  individuals.  The 
first  named  is  probably  the  most  abundant,  but  is  much  less  predomi- 
nant than  in  the  granitic  phase  on  account  of  the  greater  amount  of 
free  albite  and  oligoclase.  The  orthoclase,  to  judge  from  its  typical 
specific  gravity  (2.594  at  1G°  C. ),  must  contain  considerable  soda  in 
intimate  mixture.  The  acid  oligoclase  and  albite  are  often  surrounded 
by  a  thin  mantle  of  orthoclase,  which  is  thus  later  in  origin.  Micro- 
cline  is  probablj^  jjresent  among  the  phenocrysts,  but  is  quite  rare. 

The  groundmass  is  a  hypidiomorphic  pepper-and-salt  mixture  of  the 
same  essential  minerals  as  in  phase  /.  The  diopsidic  augite,  browni 
hornblende,  and  the  biotite  are  more  abundant  than  in  that  phase, 
causing  the  specific  gravit}^  of  the  rock  to  be  higher  (here  2. 685  at 
17°  C).  The  augite,  as  in  the  phenocrysts,  seems  always  to  occur  as 
cores  in  intergrowths  with  the  jDrimary  hornblende.  Corrosion  of 
the  dark-colored  silicates  is  much  less  pronounced  than  in  the  granitic 
phase;  they  exhibit,  correspondingly  more  often,  idiomorphic  outlines. 
Free  quartz  in  the  form  of  small  interstitial  grains  occurs,  but  is  not 
so  prominent  an  accessory  as  in  phase  /.  Titanite  is  here  more 
abundant,  and  explains  the  somewhat  higher  percentage  of  TiO^  in 
the  analysis.  The  higher  MgO  is  ascribed  to  the  more  abundant 
biotite. 

The  rock  shows  no  indications  of  crushing;  we  can  not,  therefore, 
attribute  the  porphyritic  structure  to  cataclastic  processes.  The 
groundmass  has  unquestionably  crystallized  in  its  present  form  from 
an  igneous  magma.  The  order  of  crystallization  of  its  component 
minerals  is  the  same  as  in  the  green  rock.  Several  facts  favor  the 
view  that  the  feldspar  phenocrysts  belong  to  an  earlier  stage  in  the 
crystallization  than  that  which  produced  the  groundmass. 


PLATE   V. 

A,  Segregation  of  mica  and  hornblende  concentrically  arranged,  in  paisanite 
(the  same  section  of  the  segregation  also  appears  in  the  lower  right-hand  quad- 
rant of  the  micrograph  represented  in  PI.  IV,  B) ;  ordinary  light,  X  24.    (See  p.  73.) 

B,  Basic  segregations  in  nordmarkite  at  Crystal  Cascade;  one-half  natural  size. 
(Seep.  64.) 

62 


U.  S.  GEOLOGICAL  SURVEY 


BULLETIN   NO.  209,  PL.  V 


m; 


H  <^ 


(B) 


THE   MERIDEN   GRAVURE  CO. 


DALY]  MAIN    SYENITE    STOCK.  63 

Feldsijars  of  similar  nature  and  size  are  abundant  in  the  basic 
segregations  with  which  this  phase  is  richly  charged,  and  with  them 
are  granitic  groupings  of  several  individuals  separated  by  interstitial 
quartz.  The  segregations  are  mostly  composed  of  those  minerals  of 
the  groundmass  which  crj^stallize  out  in  an  early  stage  of  consolida- 
tion. The  large  feldspars  and  the  groups  would  thus  antedate  that 
groundmass.  The  feldspar  phenocrysts  which  are  mantled  with  ortho- 
clase  very  often  present  the  appearance  of  having  been  extensively 
corroded  by  the  magma  before  the  mantles  grew  about  them.  It  is 
probable,  also,  that  there  were  two  generations  of  the  bisilicates. 
The  granitic  groupings  of  large  individuals  suggest  that  the  porphy- 
ritic  structure  may  be  largely  due  to  protoclastic  action  breaking  up 
a  coarse-grained  granitic  rock  already  more  or  less  completely  solidi- 
fied in  the  conduit  when  the  somewhat  later  magma  of  the  "ground- 
mass"  was  erupted.  On  the  other  hand,  we  can  not  exclude  the 
possibility  that  this  phase  is  the  result  of  chilling,  developing  a 
porphyritic  structure  equivalent  to  that  which  may  be  seen  in  the 
endomorphic  zone  and  in  the  aj^ophyses  of  the  granitic  phase;  for  it 
is  often  impossible  to  distinguish  hand  specimens  of  the  latter  rocks 
from  typical  specimens  of  ]3hase  g.  The  problem  thus  merits  further 
inquirj^ 

One  of  the  most  peculiar  features  of  the  s^^enites  which  may  be  seen 
in  all  the  phases,  but  is  best  exemplified  in  this  particular  phase,  is 
the  presence  in  the  rock  of  numerous  dark,  roundish  spots  or  kernels. 
These  vary  from  1  millimeter  to  1  centimeter  in  diameter.  They  occur 
in  all  parts  of  the  rock,  but  are  specially  abundant  in  the  basic  segre- 
gations, which  will  be  more  fully  described  hereafter.  The  kernels 
belong  to  two  classes,  which  show  the  common  characteristic  of  a  core 
and  mantle  structure.  Within  a  relativel}^  thin  black  outer  covering  of 
felted,  often  radiall}^  arranged,  biotite  (and  less  conspicuously  horn- 
blende) there  is  a  core  of  variable  composition.  The  latter  maj^  be 
composed  entirely  of  chlorite  and  magnetite ;  of  chlorite,  magnetite, 
biotite,  and  a  uralitic  amphibole;  or  entirely  of  a  light-green  pleo- 
chroic  actinolitic  hornblende.  The  last  mentioned  is  the  commonest 
type  of  core. 

The  mantle  is  to  be  regarded  as  a  reaction  rim.  The  chloritic  cores 
are  the  product  of  the  alteration  of  augite,  probably  in  consequence  of 
metasomatic  action.  The  actinolitic  kernels  are  likewise  plainly 
derived,  but  in  no  one  of  some  twenty-five  slides  could  there  be  found 
a  remnant  of  the  original  mineral  at  the  heart  of  the  kernel.  The 
similarity  in  size  and  general  relations  between  these  and  the  chloritic 
kernels  suggest  that  augite  was  here,  too,  the  original  material  from 
which  the  hornblende  felt  was  constructed.  The  freshness  of  the 
biotite  rim,  the  absence  of  secondary  ore  and  chlorite,  and  the  com- 
plete freshness  of  the  hornblende  core  lead  to  the  conclusion  that  the 
alteration  took  place  before  or  during  the  consolidation  of  the  rock. 


64  GEOLOGY    OF    ASCUTNEY    MOUNTAIIST,   VEEMONT.         [bull. 209. 

This  class  of  kernels  would  thus  fall  into  the  class  of  magmatic  j  * 
domorphs  after  pyroxene.  One  is  reminded  of  the  analogous  oce  "f 
alteration  of  olivine  into  hornblende. 

Chemically  this  phase  is  practically  identical  with  the  green  gran- 
ular I'ock  (cf.  columns  1  and  2  in  Table  VII).  Mineralogically  the 
similarity  is  almost  as  close.  The  only  important  difference  is  in  the 
structure.  Phase  g  may  then  be  classified  as  a  nordmarkite  with  a 
porphyritic  habit. 

BASIC   SEGREGATIONS. 

Every  observant  visitor  to  Ascutney  is  struck  by  the  extreme  rich- 
ness of  the  Main  syenite  stock  in  basic  inclosures  of  generally  a 
nodular  form,  and  he  might  also  note  that  they  are  more  abun- 
dant in  the  porphyritic  phase  than  elsewhere  (PI.  V,  B  and  PL  VI). 
They  are  distributed  with  great  irregularity.  Sometimes  they  occupy 
as  much  as  one-half  of  the  volume  of  the  rock,  if  one  may  judge  from 
the  appearance  of  even  broad  ledges.  At  other  times  the  nodules  are 
separated  by  many  feet  or  yards  of  the  normal  rock.  Partly  on 
account  of  their  abundance  in  erratics  won  from  the  mountains  a 
well-defined  glacial  bowlder  train  has  been  shown  by  C.  H.  Hitch- 
cock to  exist  in  the  lee  of  Mount  Ascutney."  The  nodules  are  dark 
gray  to  dark  greenish  gray  in  color,  spheroidal  or  ellipsoidal  in 
shape  as  a  rule,  and  of  all  sizes  up  to  those  occupying  several  cubic 
feet.  The  section  of  one  of  them,  outcropping  near  the  contact  with 
the  granite  at  the  southeast  end  of  the  mountain,  was  found  to  meas- 
ure 2  by  10  feet.  While  the  much  darker  color  causes  the  nodules  to 
be  in  striking  contrast  with  the  normal  rock  in  hand  specimen  or  in 
ledge,  microscopic  study  proves  an  intimate  dovetailing  and  interlock- 
ing of  the  minerals  between  the  two.  The  nodule  has  not  been 
enriched  in  the  bisilicates  by  the  special  impoverishment  of  the 
matrix  immediately  surrounding,  for  in  no  case  could  there  be  found 
a  zone  about  the  nodule  distinctly  lighter  in  color  than  the  normal 
rock.  This  is  the  more  difficult  to  understand  because  of  the  very 
evident  lack  of  flow  structure  in  the  rock  as  a  whole.  The  nodules 
seem  to  have  formed  quietly  in  the  magma  after  it  had  suffered  its 
"mise  en  place"  and  not  to  have  been  disturbed  in  position  since. 
Even  those  which  are  decidedly  elongated  do  not  shoAv  the  degree  of 
common  orientation  which  we  should  expect  if  they  had  floated  in  a 
streaming  fluid  matrix. 

The  nodular  masses  are  themselves  porphyritic.  Large,  irregularly 
bounded  crystals  of  microperthite,  cryptoperthite,  microcline,  ortho- 
clase  and  plagioclase  (averaging  acid  labradorite  AbiAnJ,  green  horn- 
blende, and  the  usual  diopsidic  augite,  with  or  without  a  hornblende 
mantle,  form  the  phenocrysts  (see  PL  V,  B).  The  dark  matrix  is  a 
fine-grained  granular,  panallotriomorphic  mass  of  hornblende,  oligo- 

aBuU.  Geol.  Soc.  Am.,  Vol.  I,  1890,  p.  30. 


DALY.]  BASIC    SEGEEGATIONS    IN    MAIN    SYENITE.  65 

clase,  biotite,  and  quartz,  with  abundant  grains  or  idioraorphic  crys- 
tals of  titanite,  ilmenite,  apatite,  and  zircon.  The  microperthite  and 
labradorite  of  the  phenocrysts  and  the  hornblende,  biotite,  and  oligo- 
clase  of  the  groundmass  are  really  the  essential  constituents.  The 
pleochroism  of  the  hornblende  seems  to  indicate  that  it  is  less  alka- 
ine  than  the  amphibole  of  the  matrix. 

a,  pale  yellowish  green. 

b,  grayish  green  (absorption  medium  to  strong). 

c,  grass-green  to  leek-green  (absorption  medium  to  strong). 
b  =  c  >  a. 

As  already  noted  above,  these  segregations  characteristically  contain 
light-colored,  coarsely  crystalline  areas  from  1  to  2  centimeters  in 
diameter,  similar  in  composition  to  the  normal  syenite  of  phase  /,  i.  e. 
equigranular  aggregates  of  alkali-feldspar  and  bisilicates.  These 
have  the  appearance  of  having  functioned  as  centers  of  crystallization 
during  the  growth  of  the  nodule,  although  there  is  a  complete  absence 
of  both  radial  and  concentric  structure  in  the  nodules.'^'  In  no  case 
was  there  observed  an  approach  to  the  "Kngelstruktur"  of  the  rock 
at  Virvik  or  at  the  well-known  Corsican  localit3\ 

The  specific  gravity  of  the  average  segregation  is  near  2.850,  and  is 
thus  considerably  higher  than  that  of  the  matrix,  and  still  higher  than 
that  of  the  molten  magma  which  represented  the  yet  uncrystallized 
matrix.  Unless  that  matrix  possessed  a  high  degree  of  viscosity  dur- 
ing the  formation  of  the  nodules,  they  must  have  sunk  down  in  the 
magma,  and  we  might  expect  to  find  them  concentrated  in  the 
lower  part  of  the  conduit ;  yet  they  appear  to  be  distributed  in  about 
equal  average  proportion  in  all  parts  of  the  stock  where  the  jiorphy- 
ritic  phase  was  found,  whether  at  the  summit  or  2,000  feet  vertically 
below.  This  fact  agrees  with  the  absence  of  flow  structure  in  the 
rock  in  forcing  us  to  take  the  view  that  the  segregations  do  not  belong 
to  the  preemptive  i)eriod,  as  advocated  bj'^  Lacroix,  Michael  Levy, 
Graber,  and  others  for  other  occurrences.  The  nodules  had  best  be 
referred  to  an  early  stage  in  the  actual  consolidation  of  the  syenite 
alread}'^  occupying  its  conduit. 

The  accompanying  Table  VIII  (col.  1)  shows  the  analysis  of  the 
average  segregation  from  phase  g  (spec.  66).  Columns  3,  4,  and  5 
give,  for  purposes  of  comparison,  the  analyses  of  classic  essexite, 
classic  monzonite,  and  an  average  diorite. 

The  essential  mineralogical  composition  of  these  rocks  is  as  follows: 

1.  Oligoclase,  microperthite.  cryptoperthite,  acid  labradorite,  microcline,  ortho- 
clase.  hornblende,  biotite,  augite. 

2.  Microperthite,  hornblende,  ortlioclase. 

3.  Labradorite,  orthoclase  (nepheline),  augite,  biotite,  barkevikitic  hornblende 
(olivine). 

4.  Orthoclase,  oligoclase,  andesine,  labradorite,  augite,  green  hornblende, 
biotite. 

aCf.  Chrustsehoff,  Mem.  Acad.  imp.  sci.  St.  Petersboui-g,  Ser.  VII,  Vol.  XLII,  No.  3, 1891,  p  86. 

Bull.  209—03 5 


66  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,  VEBMONT. 

Table  VIII. — Analyses  of  basic  segregations,  etc. 


[BULL.  209. 


1. 

2. 

3. 

4. 

5. 

SiOj              

56.  51 
16.  59 
1.35 
6.59 
2.52 
4.96 
5.15 
3.05 
0.71 
0.21 
0.33 
1.20 
0.04 
0.41 
0.07 
0.24 
0.06 
Trace? 
0.24 
0.03 
Trace 
Trace 

56.53 

16.47 
1.58 
5.40 
2.67 
4.90 
5.59 
3.80 
0.60 
0.23 
0.05 
1.40 
0.03 
0.27 
0.07 
0.19 
Trace 
Trace 
0.20 
Trace 
Trace 
Trace 

47. 94 
17.44 
6.84 
6.51 
2.02 
7.47 
5.63 
2.79 

1     2.04 

55.88 

18.77 

[    «8.20 

2.01 
7.00 
3.17 
3.67 

1.25 

56.52 

AI2O3             -  - 

16.31 

Fe„Oo                    

t       4.28 
1       5. 92 

FeO                            

MgO 

4.32 

CaO 

6.94 

ISTajO                                     

3.43 

K2O          

1.44 

H2O  above  110°  C 

H2O below  110°  C ---- 

CO, 

1.03 

TiO,  -     -   

0.20 

0.25 

ZrO.,- 

P„0=  -                    -  -  - 

1.04 

0.40 

CI                                  ---- - 

F                                                

FeS,                                                  

NiO,  CoO    .       

MnO    -                               

BaO                                            -   

SrO 

LiaO 

0-F,Cl-          

100.26 
0.11 

99.98 
0.09 

99.92 

99.95 

100. 98 

100. 15 
0.03 
2.849 

99.89 
Trace 

2. 756 

Total  S 

Sp.  gr 

o  With  MnO. 

1.  Basic  segregation  in  phase  g  of  Main  nordmarkite  stock;  analysis  by  Hille- 
brand. 

2.  Basic  segregation  in  dike  of  liornblende-paisanite,  Ascutney  Mountain; 
analysis  by  Hillebrand. 

3.  Classic  essexite,  Salem  Neck,  Salem,  Mass.;  analysis  by  Dittrich. 

4.  Average  analysis  of  monzonite,  according  to  Brogger,  Die  Eriiptivgesteine 
des  Kristianiagebietes,  Vol.  II,  1895,  p.  39. 

5.  Average  analysis  of  sixteen  typical  diorites,  according  to  Brogger,  ibid., 
p.  37. 

Tliere  is  a  relationship  of  the  segregation  with  each  of  these  types, 
though  that  with  monzonite  is  the  closest.  The  writer  has  seen  in 
the  laboratory  of  M.  Fouque,  in  Paris,  thin  sections  of  monzonite  from 


DALY.]  BASIC    SEGKEGATIONS    IN    MAIN    SYENITE.  67 

Predazzo,  made  from  a  contact  phase  of  that  rock.  The  structure  and 
the  small  proportion  of  monoclinic  feldspar  showed  close  similarity 
of  this  phase  of  the  monzonite  with  the  Ascutney  segregations. 

Two  other  phases  of  the  Main  stock  may  be  noted,  not  only  on 
account  of  their  importance  in  the  field  but  also  because  they  repre- 
sent interesting  extremes  in  the  differentiation  of  the  syenitic  magma. 

Phase  li  outcrops  extensively  in  a  belt  about  400  yards  (366  meters) 
wide  and  adjacent  to  the  contacts  with  diorite  and  gneiss  on  the  north- 
west side  of  the  mountain.  It  is  probably  a  special  part  of  the  endo- 
morphic  zone  of  the  stock,  as  the  phase  has  not  been  found  anywhere 
else  than  in  the  belt  specified  (spec.  34).  This  member  of  the  rock 
body  is  composed  essentially  of  microperthite,  usually  in  Carlsbad 
twins,  and  an  amount  of  quartz  sufficient  to  place  the  rock  among  the 
granites.  Biotite,  diopside,  zircon,  and  magnetite  are  the  accessories, 
but  make  up  probably  no  more  than  1  per  cent  of  the  rock.  One  grain 
of  garnet,  another  of  what  is  doubtless  corundum,  two  individuals  of 
a  brown  hornblende,  and  a  few  needles  of  apatite  were  discovered  in 
the  two  sections  that  have  been  prepared  from  this  iDhase. 

The  original  color  of  the  rock  is  due  to  the  feldspar  and  is  a  striking 
dark  oil  green,  which  is  permanent  in  the  hand  specimen,  and  doubt- 
less represents  a  late  stage  in  the  series  of  color  changes  already 
described  for  the  quarry  rock.  The  structure  is  often  fluidal  or 
trachytic  as  governed  by  the  tabular  feldspars. 

To  form  an  idea  of  the  relative  proportions  of  the  soda  and  potash 
molecules  in  the  rock  the  specific  gravity  of  some  thirty  cleavage  pieces 
of  the  feldspar  was  determined.  Specific  gravity  could  be  safely  relied 
upon  on  account  of  the  freshness  of  the  rock  and  on  account  of  the  lack 
of  inclusions  in  the  feldspar.  The  average  for  the  thirtj^  X)i6ces  was 
2.594  at  22°  C. ;  the  range  of  specific  gravity  was  from  2.582  to  2.612. 
The  extinction  angles  showed  that  the  albite  of  the  intergrowth  is 
nearly  pure  and  has  only  a  very  small  intermixture  with  the  lime  mole- 
cule. Accepting  Brogger's  values  for  the-specific  gravities  of  pure 
albite  and  pure  orthoclase  the  average  for  this  rock  corresponds  to  a 
microperthite  in  which  the  two  silicates  occur  in  about  equal  propor- 
tion, with  the  albite  the  more  abundant.  The  specific  gravity  of  the 
rock  is  2.616  at  the  same  temperature.  If  we  assume  that  the  ratio 
Ab:  Or=41: 35  as  in  the  granitic  phase,  that  the  lime  is  1  per  cent  of 
the  rock  and  the  accessories  1  per  cent,  there  would  be  about  20 
per  cent  quartz  in  the  rock.  This  rough  estimate  agrees  with  that 
made  by  inspection,  of  the  thin  sections.  This  phase  is  thus  a  true 
alkaline  granite  at  the  extreme  end  of  the  series  which  leads  to  a  rock 
with  the  composition  of  an  aplite  while  preserving  the  hypidiomorphic- 
granular  structure.  A  very  similar  rock  occurs  near  Stratford,  N.  H. , 
Albany,  N.  H.,  and  Stark,  N.  H.  These  are  illustrated  in  the  collec- 
tion of  Professor  Rosenbusch  at  Heidelberg.  They  all  possess  a 
higher  prox3ortion  of  bisilicates  than  phase  h. 


68  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VEEMONT.         [bull. 209. 

Another  variety  of  true  granite,  into  which  the  syenite  is  transi- 
tional, outcrops  at  the  main  summit  of  the  mountain.  It  has  the 
ordinary  pinkish  color  of  the  average  syenite  of  the  stock.  The  com- 
position is  essentially  the  same  as  that  just  described  for  phase  li. 

Phase  i. — Near  the  most  westerly  triple  contact  of  granite  stock, 
syenite  stock,  and  phyllites,  a  fourth  phase  of  the  syenite  was  speedily 
noted  in  the  field  as  unlike  all  the  others  in  bearing  an  unusual 
amount  of  dark-colored  minerals  (spec.  111).  The  light-gray  feld- 
spars still  give  the  dominant  tone  of  color  to  this  phase,  which  is  als© 
alkaline.  The  structure  is  that  of  phase/,  equigranular;  the  grain 
is  somewhat  coarser.  Basic  segregations  fail  altogether  or  are  very 
rare.  The  chief  mineralogical  difference  between  this  phase  and  the 
others  is  found  in  the  character  of  the  feldspars.  Triclinic  feldspar 
is  now  one  of  the  chief  essentials.  It  varies  in  composition  from  the 
labradorite  Ab3An4  to  the  andesine  Ab4Ang.  Microperthite  and  ortho- 
clase,  hornblende,  diopside,  and  biotite,  with  the  properties  of  these 
minerals  in  phase  /,  are  the  other  essentials ;  the  same  accessories  are 
found  here  excepting  allanite,  monazite,  and  garnet.  In  addition 
there  is  a  second  hornblende  among  the  essentials,  with  the  following 
scheme  of  pleochroism  and  absorption: 

a,  Yellowish  brown. 

h,  Deep  brown  to  black,  with  specially  strong  absorp- 
tion. 

c,  Deep  brown,  with  a  trace  of  olive-green. 
b  >  c  >  a. 

The  augite  occurs  only  as  cores  in  intergrowth  with  hornblende. 
Quartz  is  very  subordinate  among  the  accessories. 

The  basic  character  of  this  i^hase,  its  richness  in  bisilicate,  the 
presence  of  much  essential  andesine-labradorite,  coupled  with  the 
alkaline  habit  of  the  rock,  are  properties  which  relate  the  rock  closely 
to  nionzonite.  The  resemblance  of  the  Ascutney  hand  specimens  and 
those  from  the  classic  locality  of  Mount  Mulatto  is  ver^^  striking. 

ENDOMORPHIC    ZONE    OF   THE    SYENITE    STOCK. 

All  four  phases  of  the  stock  are  habitually  more  acid  near  the  contact 
than  elsewhere.  Free  quartz  is  even  macroscopically  so  dominant  that 
the  contact  rock  stamps  itself  as  a  true  granite.  The  usual  chilling 
phenomena  occur  within  a  narrow  zone  not  more  than  20  feet  across. 
Three  feet  from  the  contact  the  feldspar  and  quartz  become  idiomori^hic 
and  are  embedded  in  a  microcrystalline,  often  granophyric,  ground- 
mass,  and  a  granite-porphyry  is  thus  developed.  Apophyses  fail  to 
show  as  much  bisilicate  as  the  parent  rock  body,  have  more  free  quartz, 
and  tend  toward  the  structure  of  a  typical  aplite.  Excellent  examples 
may  be  studied  among  some  thick  sheets  on  the  north  slope  of  the 
mountain,  below  the  quarry.     This  endomorphic  increase  of  silica  is 


DALY.]  DIKE    AT    LITTLE    ASCUTNEY    MOUNTAIISr.  69 

paralleled  in  the  Christiania  region,  where  angite  granite  occurs  as 
the  contact  phase  of  angite  syenite  rich  in  nepheline.'*- 

GREAT   SYENITE-PORPHYRY   DIKE  OF  LITTLE  ASCUTNEY 

MOUNTAIN. 

The  stock-like  dike  to  which  Little  Ascutney  chiefly  owes  its  exist- 
ence is  remarkable  for  following  in  its  general  east-west  course  the 
zone  of  contact  between  the  diorite  and  gneiss  (PI.  VII  and  fig.  1). 
Throughout  its  whole  extent  the  south  wall  of  the  dike  is  schist  and  the 
north  wall  is  either  diorite  or  the  green  dike  mapped  as  "'  paisanite." 
This  replacement  of  the  zone  of  contact  rocks  by  a  later  intrusive  is 
undoubtedly  due  to  the  weakness  of  that  zone,  which  is  elsewhere 
evident  in  the  extensive  brecciation  and  crumpling  of  the  schistose 
rock.  Small  apophyses  from  the  dike  into  both  schist  and  diorite 
clearly  prove  the  syenite-porphyry  to  be  the  younger  rock. 

The  dike  is  quite  uniform  in  composition  from  end  to  end,  though 
there  is  a  coarsening  of  grain  from  the  walls  toward  the  center.  The 
rock  is  almost  the  exact  equivalent  of  the  average  porphyritic  phase 
of  the  Main  syenite  stock,  and  a  detailed  description  is  therefore 
unnecessarj^  (spec.  76).  Hornblende  and  biotite  are  the  essential 
dark-colored  minerals.  They  and  the  feldspars  again  display  a  great 
amount  of  magmatic  corrosion;  this  caustic  action  is  here,  as  so  often 
elsewhere  in  alkaline  rocks,  responsible  for  the  rarity  of  idiomorphic 
boundaries  among  the  phenocrysts,  as,  indeed,  it  may  be  responsible 
for  the  general  rarity  of  i^orphyritic  dike  representatives  of  the  alka- 
line magmas  as  a  whole.  Plagioclase  seems  to  be  entirely  absent 
from  this  rock  except  in  the  form  of  a  few  rare  phenocrysts  of  an 
acid  oligoclase.  Microperthite  is  not  so  abundant,  either  in  the 
groundmass  or  among  the  phenocrysts,  as  it  is  in  the  porphyritic 
phase  of  the  Main  syenite  stock. 

The  dike  is  to  be  classified  as  a  nordmarkite-porphyry . 

Dark  patches  of  basic  material  are  very  common  in  this  dike. 
They  are  roundish  in  form  and  vary  from  a  fraction  of  an  inch  to  3 
or  4  inches  in  diameter.  In  color  and  general  nnacroscopic  appear- 
ance they  are  similar  to  the  nodules  from  phase  g  in  the  Main  stock. 
The  correspondence  is  more  fully  shown  in  the  thin  section.  The 
same  constituents  are  present  as  in  the  typical  segregation  of  the 
Main  stock  and  in  the  same  relative  amounts.  That  exception  is 
significant.  Both  in  the  phenocrystic  constituents  and  in  the  ground- 
mass  of  the  nodule  microperthite  is  not  so  abundant  as  in  the  nodules 
from  phase  g.  This  fact  indicates  another  proof  of  close  sympathy 
between  nodule  and  host  and  of  the  indigenous  origin  of  the  latter. 
The  nodules  themselves  add  another  evidence  to  the  community'  of 
origin  between  this  dike  and  the  Main  stock.  There  can  be  little 
doubt  that  the  two  intrusions  were  products  of  essentially  contem- 
poraneous eruptions  from  a  common  magma. 


o  Zeit.  fiir  Kryst.,  Vol.  XVI,  1890,  p  327. 


70  GEOLOGY    OF    ASCUTNEY   MOUNTAIN,   VERMONT.         [bull.  209. 

SYENITE  STOCK  OF  PIERSON  PEAK. 

The  same  may  be  said  of  the  small  stock  of  Pierson  Peak.  The 
plan  of  tills  rock  body  is  elliptical.  The  longer  axis  measures  400 
yards  (366  meters),  running  about  N  70°  E;  the  minor  axis  measures 
175  yards  (160  meters).  Coarse-grained  apophyses  prove  the  intru- 
sive origin  of  the  rock.  It  is  uniformly  a  coarse-grained,  light  gray 
to  light  pinkish-gray  alkaline  syenite,  with  a  proportion  of  dark 
colored  constituents  which  is  low  even  in  comparison  with  the  phases 
of  the  Main  stock  (spec.  62).  The  structure  is  the  typical  hypidio- 
morphic-granular,  the  order  of  crystallization  that  of  phase/  of  the 
Main  stock.  Microperthite,  orthoclase,  and  biotite  are  the  essentials. 
Quartz,  hornblende,  augite,  apatite,  and  zircon  are  all  notably  rare 
accessories;  more  important  are  titaniferous  magnetite  and  titanite, 
the  latter  being  unusually  abundant.  Basic  segregations  fail  alto- 
gether or  are  extremelj^  rare.  The  endomorphic  zone  is  characterized 
by  an  almost  complete  lack  of  colored  constituents  and  of  quartz. 

All  the  minerals  have  the  same  characters  as  in  the  granular  phases 
of  the  Main  stock.  The  rock  is  a  nearly  quartzless  biotite-nord- 
markite,  or  pulaskite.  The  writer  proposes  that  the  existing  diffi- 
culty of  differentiating  these  two  rock  types  (compare  cols.  4  and  5, 
Table  VII,  p.  59)  be  obviated  by  confining  the  name  "pulaskite"  to  a 
rock  which  is  in  all  other  respects  the  equivalent  of  the  nordmarkitas 
except  in  the  absence  or  subordination  of  accessory  free  quartz  among 
the  constituents.  Excepting  for  a  higher  proportion  of  bisilicates  in 
the  Arkansas  syenite,  it  would  be  hard  to  distinguish  macroscopically 
this  Ascutne}^  rock  from  the  classic  pulaskite  of  Fourche  Mountain; 
there  is  similar  close  parallelism  with  a  geographically  remote  occur- 
rence of  the  same  type — that  of  Portella  des  Eiras  at  Monchique, 
Portugal. 

APIjITIC  dikes  CUTTIIS^G  THJE  SYEIflTES. 

Three  kinds  of  acid  dikes  have  been  found  cutting  the  various  sye- 
nites of  the  area.  Two  of  these  are  intimately  related  to  the  stock 
phases;  the  third  has  variant  features.  It  may  be  noted  that  there 
is  an  unusual  lack  of  pegmatite  veins  both  in  the  syenites  and  else- 
where about  Ascutney, 

PAISANITE  DIKE  CUTTING  THE  MAIN  STOCK. 

On  the  logging  road  running  up  from  a  sawmill  on  the  northwest 
slope  of  Ascutney  Mountain  proper,  toward  the  main  summit,  a  dike 
was  discovered  in  the  dark-green  granular  phase /of  the  Main  stock 
at  about  the  1,600-foot  contour  (see  PI.  VII).  The  general  trend  of  the 
dike  is  northeast-southwest,  but  at  the  road  it  bifurcates  into  two 
branches^ — ^one,  40  feet  (12  meters)  wide,  striking  N,  40""  E. ;  the  other, 
50  feet  (15  meters)  wide,  striking  N.  25"  E.  The  dike,  as  a  whole,  is 
visible  only  for  about  100  yards  (91  meters);  at  each  end  its  continua- 
tion is  lost  in  the  underbrush  and  talus  of  the  steep  mountain  side. 


^|-    llXi-i- 


111 


iD-|[jiCiUfflLiart|fBiii 


DALY.]  PAISANITE    DIKE    CUTTING    MAtN    STOCK.  71 

The  rock  is  alight-tinted,  pinkish-gray,  pepper-and-salt,  fine-grained, 
somewhat  porphyritic  aggregate  of  microperthite,  soda-orthoclase, 
quartz,  and  alkaline  hornblende,  abundantly  charged  with  basic  seg- 
regations, with  kernels  of  biotite  and  hornblende,  and  with  pyroclastic 
feldspars  won  from  the  coarse  syenite  through  which  the  dike  passed 
during  intrusion  (spec.  139).  The  general  habit  is  suggestively  like 
that  of  the  porphyritic  phase  g  of  the  Main  stock,  and  we  must  believe 
that  the  two  are  products  of  the  same  magma.  Yet,  as  we  have  seen, 
the  implication  that  phase  /  and  phase  g  are  of  different  ages  (the 
former  being  cut  by  the  latter)  does  not  agree  with  the  fact  of  obser- 
vation in  the  field.  It  is  probable  that  there  was  not  a  great  interval 
of  time  between  the  intrusion  of  the  Main  stock  and  that  of  this  dike. 

The  dike  is  characterized  by  a  conspicuous  platy  structure  due  to 
jointing,  and,  near  its  walls,  exhibits,  for  the  space  of  a  foot  or  two 
from  the  contacts,  a  strong  fluidal  character  which  is  the  more  pro- 
nounced as  the  basic  segregations  have  shared  in  the  movement  and 
are  pulled  out  in  long,  dark-colored  streaks  in  the  dike. 

The  phenocrysts  are  either  microperthite  or,  more  rarely,  ortho- 
clase;  they  are  specially  abundant  and  are  difficult  to  distinguish 
from  the  pyroclastic  feldspars.  The  texture  of  the  rock  is  really  con- 
trolled by  the  groundmass,  the  structure  of  which  is  aplitic  or  panal- 
lotriomorphic.  It  is  composed  of  microperthite,  quartz,  and  brown 
hornblende,  with  properties  identical  with  those  of  phase  /  in  tlie 
Main  stock.  The  hornblende  always  occurs  in  the  form  of  small, 
poikilitic,  and  greatly  resorbed  grains.  No  biotite,  diopside,  or  pla- 
gioclase  were  discoverable.  Titanite,  ilmenite,  zircon,  and  apatite 
are,  as  usual,  the  accessories. 

The  pyroclastic  feldspars  are  of  special  interest.  They  occur  as 
single  individuals  or  as  groups  (with  interstitial  quartz)  of  the  same 
structure  and  grain  as  the  country  rock  of  the  dike  itself.  Close  study 
in  the  field  showed  conclusively  that  thej^  are  of  pyroclastic  origin. 
Their  presence  in  the  still  unconsolidated  dike  affected  its  crystalli- 
zation, so  that  many  of  these  foreign  feldspars  are  surrounded  by  typi- 
cal reaction  rims  of  material  considerably  more  basic  than  the  aver- 
age groundmass  of  the  dike.  The  usual  appearance  of  the  feldspar 
inclosure  with  its  basic  aureole  is  illustrated  in  PL  IV,  B  (p.  58).  The 
feldspar  in  this  case  is  soda-orthoclase,  and  in  the  reaction  of  the  exten- 
sively corroded  feldspar  with  the  matrix,  the  bisilicate  of  the  reaction 
rim  is  even  more  strongly  charged  with  soda  than  the  brown  horn- 
blende of  the  average  groundmass.  The  rim  is  an  interlocking  aggre- 
gate of  microperthite  and  amphibole,  with  abundant  magnetite  and 
some  zircon  and  apatite.  The  amphibole  has  bluish  tones,  as  indica- 
ted by  the  scheme  of  pleochroism  and  absorption. 

a,  brownish  yellow. 

b,  deep  blue-green. 

c,  deep  chestnut-brown,  with  a  tinge  of  blue  on  the  edges. 
l)>c>a,  or  perhaps  b=c>a. 


72  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 

The  extinction  c:  c  is  about  19°. 

This  same  hornblende  forms  minute  basic  segregations  varying  in 
size  up  to  1  or  2  millimeters  in  diameter.  These  lie  in  the  general 
groundmass  of  the  dike,  and  are  not  directly  connected  with  the  pyro- 
clastic  feldspars.  Other  segregations  or  replacements  which  recall 
the  "kernels"  of  the  Main  syenite,  and  yet  show  somewhat  different 
composition  and  structure,  also  occur  in  the  groundmass.  One  of 
these  is  illustrated  in  PI.  V,  A  (p.  62).  It  is  composed  entirely  of  a 
faintly  pleochroic,  yellow  to  light-brown,  biotitic  mica  arranged  in 
alternating  concentric  zones  with  a  blue  hornblende.  The  pleochro- 
ism  of  the  latter  is : 

a,  pale  straw-yellow. 

b,  deepish  green-blue. 

c,  blue,  with  a  trace  of  green. 
c  about=b>a. 

The  origin  of  these  concentrically  arranged  mica-hornblende  aggre- 
gations has  not  yet  been  determined.  Their  resemblance  to  the  ker- 
nels of  the  Main  syenite,  which  have  been  interpreted  as  magmatic 
pseudomorphs,  is  only  partial.  Especially  difficult  of  understanding 
is  the  recurrence  of  the  zones  of  mica  and  hornblende. 

BASIC    SEGREGATIONS. 

The  usual  basic  segregation  of  the  dike  is  very  similar  to  that  in  the 
porjDhyritic  phavse  of  the  Main  syenite  (spec.  141).  It  is  a  dark-gray, 
mottled  aggregate  of  phenocrysts  and  p}.  roclastic  feldspars  surrounded 
by  a  dense,  granular  groundmass  of  panallotriomorphic  brown  horn- 
blende, microperthite,  and  orthoclase.  Here  there  can  be  no  doubt 
that  the  segregation  grew  under  the  directing  influence  of  the  large 
feldspars  now  seen  within  their  mass,  for  the  segregation  of  basic 
material  is  decidedly  more  pronounced  in  the  immediate  vicinity  of 
the  feldspars  than  elsewhere  in  the  nodules.  The  nodules  vary  in 
size  up  to  2  or  3  inches  (5.2  to  7.8  cm.)  in  diameter.  In  one  slide  a 
large  crystal  of  pale-green  augite  with  a  mantle  of  green  hornblende 
was  found,  suggesting  that,  after  all,  the  kernels  of  this  rock  may 
have  been  derived  from  that  mineral  through  magmatic  influences. 
The  hornblende  of  these  larger  segregations  has  a  bluish  cast,  and 
seems  to  belong  to  a  variety  of  amphibole  intermediate  between  the 
hornblende  of  the  reaction  rim  described  above  and  the  normal  horn- 
blende of  the  Main  syenite.  The  specific  gravity  of  a  typical  segre- 
gation was  found  to  be  2.756;  that  of  the  parent  rock,  2.633. 

The  chemical  analysis  of  the  average  matrix  in  which  the  basic 
segregations  lie  is  given  in  the  Table  IX,  col.  1,  p.  75;  that  of  an  aver- 
age segregation  from  the  dike  is  entered  in  Table  VIII,  col.  2,  p.  66. 
The  structure  and  composition,  both  mineralogical  and  chemical, 
relate  the  dike  most  intimately  with  paisanite,  as  described  by  Osann 


DALY]        PAISANITE    DIKE    ON    LITTLE    ASCUTNEY    MOUNTAIN.  73 

(see  Table  IX,  column  5).  If  we  assume  that  there  is  5  per  cent  of 
soda  in  the  hornblende,  and  that  that  mineral  makes  up  3  per  cent  of 
the  rock  (a  fair  estimate  after  insijection  of  the  slide),  the  mineral  com- 
position of  the  rock  can  be  thus  roughly  determined  as  the  following: 

Per  ceut. 

Albite  molecule 36. 4 

Orthoclase  molecule 29. 6 

Quartz 29. 5 

Hornblende 3.0 

Titanite , : .5 

Other  accessories 1.0 

100 

The  analysis  of  the  segregation  does  not  lend  itself  to  calculation 
on  account  of  the  abundance  of  the  hornblende,  the  constitution  of 
which  is  unknown.  The  essential  equivalence  of  this  analysis  and 
that  of  the  basic  segregation  from  phase  g  of  the  Main  syenite  is 
striking.  Again,  among  normal  rocks,  we  must  go  to  the  monzo- 
nites  for  the  nearest  allies,  chemicallj'  speaking,  to  these  nodular 
masses.  Mineralogically,  the  greatest  difference  between  segregation 
and  its  matrix  is  found  in  the  absence  of  triclinic  feldspar  in  the 
former. 

COMMON   MUSCOVITE-APLITE  OF  THE    MAIN   STOCK. 

Due  south  of  the  Windsor  quarry,  at  the  2,350-foot  contour,  a  dike 
about  1  foot  (0.3  meter)  in  diameter  traverses  the  syenite  at  a  point 
where  the  latter  is  porphyritic  and  full  of  segregations.  No  other 
dike  of  the  same  composition  has  been  discovered  in  the  area,  but  it 
is  highly  probable  that  others  exist.  This  dike  is  a  typical  aplite,  a 
panallotriomorphic  sugary  mixture  of  quartz,  orthoclase,  and  albite, 
with  a  little  micro]3erthitic  feldspar  and  a  few  shreds  of  muscovite 
(spec.  191).  The  last  mentioned  mineral  occujpies  certain  areas  in 
the  thin  section  as  if  it  is  secondarj^  after  feldspar;  in  other  cases, 
patches  of  matted  quartz  and  muscovite  represent  the  filling  of  small 
miaroles  which  are  common  in  the  rock.  Numerous  miarolitic  cavi- 
ties bearing  terminated  quartz  crystals  and  muscovite  plates  are 
visible  in  the  hand  specimen. 

PAISANITE  DIKE  ON   LITTLE  ASCUTNEY  MOUNTAIN. 

On  referring  to  the  plan  of  Little  Ascutney  intrusives  (fig.  1)  it 
will  be  seen  that  there  is  intercalated  between  the  great  syenite- 
porphyry  dike  of  that  ridge  and  the  diorites  on  the  north  a  second 
interrupted  dike,  which  thus  entered  the  same  zone  of  weakness  at 
the  schist-contact  as  that  earlier  followed  by  the  porphyry.  This 
second  dike  is  much  smaller  than  the  first,  but  measures,  neverthe- 
less, about  50  yards  (46  meters)  in  width  at  the  broadest  part.  It  isj 
probably  this  rock  that  was  referred  toby  Hawes  as  the  "granitell 


74  GEOLOGY    OF    ASCUTNEY    MOUNTAIJSr,   VERMONT.         [bull.  209. 

of  Little  Ascutney."**  It  sends  apophysal  tongues  into  the  diorite 
and  is  similarly  believed  to  be  younger  than  the  porphyry,  although 
only  on  account  of  the  chilling  phenomena  observed  in  hand  speci- 
men and  slide  from  the  smaller  dike  where  it  is  in  contact  with  the 
other. 

Hints  as  to  the  relationship  of  this  dike  are  to  be  found  in  the  ledge 
and  hand  specimen.  When  quite  fresh  the  rock  is  a  fine  pale  gray 
with  a  blue  tone;  in  a  few  days  it  changes  color  to  the  same  hand- 
some olive-gray  green  which  has  been  described  as  characteristic  of 
the  Windsor  quarry  rock.  The  resemblance  between  the  two  rocks  is 
also  manifest  in  the  way  in  which  they  fracture,  and  in  the  peculiarly 
vibrant  musical  note  given  out  when  a  large  fragment  is  struck  with 
the  hammer.  Certain  of  the  finer-grained  streaks  in  the  quarry  can 
hardly  be  distinguished  from  the  green  dike  in  the  hand  specimen. 


?"^?3^ 


CamptonitediKes       Paisanite      Nordmarkite-porphyry   Diorite  Breccia 

Fig.  1. — Sketcli  plan  of  intrusive  rocks  on  Little  Ascutney  Mountain. 

The  rock  is  a  very  fine-grained  typical  aplite  with  a  sugary,  panal- 
lotriomorphic  structure  (spec.  60).  The  essential  minerals  are  nearly 
the  same  as  in  the  paisanite  just  described  from  the  main  mountain. 
Quartz  is,  however,  quite  prominent  among  the  phenocrystic  indi- 
viduals which  are  otherwise  composed  of  microperthite,  either  in 
separate  crystals  or  in  groups.  The  same  constituents,  with  crypto- 
perthite  and  an  alkali-iron  hornblende  identical  in  characters  with 
that  of  the  Windsor  quarry  rock,  are  the  essentials  in  the  groundmass. 
The  quartz  and  feldspar  "  phenocrysts "  are  connected  through  all 
stages  of  transition  with  the  same  minerals  of  the  groundmass,  and  it 
is  probable  that  there  has  been  but  one  generation  of  these  essentials. 
The  hornblende  is  strikingly  poikilitic,  as  if  corroded  in  the  extreme. 
Biotite,  oligoclase,  magnetite,  apatite,  and  zircon  occur  as  accessories. 

n  Geology  of  New  Hampshire,  Vol.  Ill,  part  4, 1878,  p.  302. 


DALY.  I 


PAISANITE. 


75 


Table  IX. — Analyses  of  paisanites  and  other-  rocks. 


1. 

2. 

3. 

4. 

.5. 

6. 

SiO^  __._ 

AI2O3 

FcsOg 

73.69 

12.46 

1.21 

1.75 
0.17 
0.36 
4.47 
4.92 
0.24 
0.14 
Trace 
0.28 
0.14 
0.04 
0.02 
0.05 

73. 03 
13.43 
0.40 
1.49 
0.14 
0.79 
4.91 
4.54 
0.35 
0.18 
Trace? 
0.30 
0.06 
0.06 
0.03 
0.08 
0.09 
0.15 
Trace 
Trace 
Trace 
? 

77.14 
12.24 
0.39 
1.04 
0.06 
0.35 
4.64 
4.47 

\  «0.14 

66.50 
16.25 
2.04 
0.19 
0.18 
0.85 
7.52 
5.53 

a  0.50 

73.35 
14.  38 
1.96 
0.34 
0.09 
0.26 
4.33 
5.66 

70.19 

11.96 

4.94 

FeO 

1.18 

MgO 

CaO                   --       .       

0.16 
0.65 

NasO  

5.73 

K2O. 

4.06 

H2O  above  110°  C 

H2O  below  110°  C-     

CO2                                       

Ti02 

0.39 

0.70 

ZrOg 

p,o. 

Trace 

CI                                     

F 

FeSa 

MnO 

0.15 

Trace 

0.20 

0.48 

BaO 

SrO                                     

Faint  tr. 
Trace? 
Trace 

LijO 

CuO 

0-F,Cl 

100. 09 
0.02 

100.03 
0.04 

100.66 

100. 46 

100. 37 

99. 94 

100. 07 

99.99 
0.05 
3.628 

Total  S                              -     -  - 

Sp.  er 

2. 633 

a  Loss  on  ignition. 


1.  Hornblende-paisanite  dike  cutting  Main  syenite,  Ascutney  Mountain;  analy- 
sis by  Hillebrand. 

2.  Hornblende-paisanite  dike  cutting  nor dmar kite-porphyry,  Little  Ascutney; 
analysis  by  Hillebrand. 

3.  Lestivarite,  Bass  rocks.  Gloucester,  Essex  County,  Mass.;  analysis  by  Wash- 
ington, Jour.  GeoL.  Vol.  VII,  1899.  p.  107. 

4.  Classic    lestivarite,   Brogger,   Die    Eruptivgesteine   des  Kristianiagebietes, 
Vol.  Ill,  1898,  p.  216;  analysis  by  V.  Schmelck. 

5.  Classic  paisanite,  Osann,  Tscher.  Miner,  n.  Petrog.  Mitth.,  Vol.  XV,  1896, 
p.  439. 

6.  Average  grorudite,  according  to  Brogger,  Die  Eruptivgesteine  des  Kristiania- 
gebietes, Vol.  I,  1894,  p.  63. 


76 


GEOLOGT    OV    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 


The  chemical  analysis  of  this  rock  is  given  in  Table  IX,  column  2, 
along  with  that  of  other  rocks  of  related  types.  Their  corresponding 
essential  mineralogical  composition  is  as  follows : 

1.  Microperthite,  soda-orthoclase.  qviartz,  alkaline  hornblende. 

2.  Microperthite,  qnartz,  soda-orthoclase,  alkaline  hornblende. 

3.  Microperthite.  other  alkaline  feldspar,  quartz,  hornblende,  biotite. 

4.  Cryptopertliite,  segirine. 

5.  Microperthite,  cryptoperthite,  quartz,  riebeckite. 

6.  Quartz,  microperthite,  microcline,  albite,  soda-orthoclase,  segirine,  catoforite. 

The  molecular  jjroportions  for  the  Ascutney  rock  have  been  calcu- 
lated as  follows: 


Analysis. 

Molecular 
proportions. 

SiOa 

73.03 
13.43 
0.40 
1.49 
0.14 
0.79 
4.91 
4.54 
0.30 
0.06 
0.06 

1.2165 

A1203 .      _       ..                             

.1313 

FCaOg 

.0025 

FeO 

.0207 

MgO  -       -  - 

.0035 

CaO _              _.......__ 

.0141 

Na,>0 .  _                                

.0792 

K2O . 

.0484 

TiO, .__ 

.0036 

ZrOs 

.  0005 

P2O5---- 

.0004 

If  we  suppose  that  the  hornblende  has  10  per  cent  lime,  3  per  cent 
soda,  and  40  per  cent  silica  (not  far  from  the  proportions  of  those 
oxides  in  barkevikite),  we  can  get  an  aj)proximate  idea  of  the  quan- 
titative mineral  composition  of  the  rock.  On  these  su^jpositions  the 
albite  molecule  would  make  up  40  per  cent  of  the  rock.  The  propor- 
tion of  the  same  molecule  would  be  38  per  cent  if  the  hornblende  were 
5  per  cent  soda,  and  41.5  per  cent  if  all  the  soda  were  in  the  feldspar. 
The  results  of  the  calculation,  based  on  an  accurate  knowledge  of  the 
composition  of  all  the  minerals,  would  not  be  far  from  the  following: 

Per  cent. 

Albite  molecule 40 

Orthoclase  molecule 27 

Quartz 27 

Anorthite  molecule 2 

Hornblende. -.'. 3 

Accessories 1 1 

100.0 

A  comparison  of  columns  1,  2,  and  5  in  Table  IX  shows  at  once  the 
thorough  similarity  of  this  rock  to  classic  paisanite  and  to  the  great 


DALY.]  BUECCIA    OK    LITTLE    ASCUTNEY    MOUNTAIN.  77 

aplite  dikes  on  the  northwest  side  of  the  main  monntain.  The  last 
mentioned  we  have  seen  to  be  an  acid  representative  of  the  porphy- 
ritic  phase  of  the  Main  stock;  in  the  same  way  the  Little  Ascutney 
paisanite,  in  its  composition,  evanescent  color,  and  freedom  from 
basic  segregations,  is  closely  allied  to  the  granitic  phase  of  the  same 
stock.  Both  of  the  Ascutney  paisanites  are  allied  to  grorudite  (column 
6)  and  to  the  "  lestivarite "  of  Essex  County  (column  3)  which,  how- 
ever, is  a  type  considerably  divergent  from  classic  lestivarite 
(column  4). 

BRECCIA  MASSES  ON  LITTI.E  ASCUTNEY  MOUNTAIN. 

Inclosed  in  the  green  paisanite  dike  are  2  horses  of  breccia,  the 
larger  measuring  in  plan  25  feet  (7.5  meters)  by  8  feet  (2.4  meters). 
In  the  older  adjoining  sj^enite-porphj-ry  dike  tliere  are  at  least  16 
similar  horses  exposed  on  the  crest  of  the  ridge,  as  indicated  on  the 
sketch  map  (fig.  1,  p.  74).  The  largest  of  the  horses  in  the  porphj^ry 
is  180  feet  (55  meters)  in  length  by  55  feet  (16.5  meters)  in  breadth. 
The  smallest  one  mapped  covers  8  (2.4  meters)  or  10  (3  meters)  feet 
square,  though  many  smaller  fragments  of  the  same  rock  occur 
scattered  through  the  porphyry. 

These  interesting  bodies  were  first  described  by  the  geological  sur- 
vey  of  Vermont.  In  its  final  report  Edward  Hitchcock  developed  a 
theory  of  the  Ascutney  eruptives  which  is  founded  on  the  discovery 
of  the  breccia.     It  can  best  be  expressed  in  his  own  words : 

3t-  *  *  If  yf^Q  ascend  Little  Ascutney,  near  its  west  end,  on  the  top,  just  where 
the  southern  slope  begins,  masses  of  a  conglomerate  of  a  decided  character,  several 
feet  and  even  rod's  wide,  appear  on  the  side  of  the  porphyry  and  granite.  All  traces 
of  stratification  in  the  conglomerate  are  lost  and  it  passes  first  into  an  imperfect 
porphyry,  and  this  into  granite  without  hornblende,  in  the  same  continuous  mass, 
without  any  kind  of  divisional  plane  between  them.  Where  the  conglomerate  is 
least  altered  it  is  made  up  almost  entirely  of  quartz  pebbles  and  a  larger  amount 
of  laminated  grits  and  slate,  the  fragments  rounded  somewhat  and  the  cement  in 
small  quantity.  It  is  easy  to  see  that  a  metaniorphism  has  taken  place  in  all  the 
conglomerate  and  some  of  the  pebbles  might  even  be  called  mica-schist.  In  the 
cement  also  we  sometimes  see  facets  of  feldspar.  In  short,  it  is  easy  to  believe 
that  the  process  of  change  need  only  be  carried  further  to  produce  syenite,  por- 
phyry, or  granite.  One  can  not  resist  the  conviction  that  the  granite  rocks  of  the 
mountain  are  nothing  more  than  conglomerate  melted  down  and  crystallized,  or 
at  least  that  such  was  the  origin  of  part  of  them." 

Van  Hise  has  dissented  from  this  view  and  briefly  stated  his  opinion 
that  these  "pseudo-conglomerates "  are  flow  breccias.  He  says :  "The 
matrices  of  these  rocks  are  thoroughly  crystalline  granular  granite, 
syenite,  or  porphyry.  Thus  they  are  eruptives  which  have  caught 
within  them  fragments  of  the  rocks  through  which  they  have  passed."* 

The  observations  of  the  writer  do  not  agree  with  Hitchcock's  deter- 
mination of  the  relation  between  the  horses  and  the  porphyry.    Tliere 

a  Geology  of  Vermont,  1861,  Vol.  II,  pp.  565-566. 
&Bull.  Geol.  Soc.  Am.,  Vol.  I,  p.  236,  footnote. 


78  GEOLOGY    OF    ASCUTNEY   MOUNTAIN,   VEEMONT.         [bull.  209. 

is  no  transition  between  the  two,  but  instead  a  very  clean-cut  contact, 
which  is  just  as  distinct  as  that  between  the  porphyry  and  the  schists. 
The  facts  that  lead  to  the  rejection  of  Van  Hise's  theory  of  the  breccia 
will  also  be  briefly  noted. 

The  horses  do  present  in  the  field  the  general  appearance  of  flow 
breccias  (spec.  36).  In  thin  section,  however,  the  compact,  dark 
greenish-gray  cement  resolves  itself  into  an  aggregate  of  clastic  grains 
of  quartz  and  feldspar  in  a  secondary  groundmass  of  argillaceous 
material,  chlorite,  biotite,  and  quartz.  The  biotite  looks  metamor- 
phic  and  is  concentrated  about  magnetite,  which  is  comparatively 
abundant.  Small  garnets  are  also  interspersed  in  the  cement  in  great 
numbers,  and,  like  the  biotite,  were,  in  all  probability,  formed  in  the 
partial  alteration  of  the  cement  by  the  heat  and  mineralizers  of  the 
porphyry  intrusion. 

There  can  be  no  doubt  that  such  a  matrix  is  clastic,  and  the  shape 
and  nature  of  the  inclosed  fragments  agree  with  that  interpretation. 
They  are  subangular  or  angular  and  without  visible  stratification. 
In  size  they  vary  from  those  of  microscopic  dimensions  to  others  hav- 
ing an  area  of  a  square  foot  or  more.  Usually  the  corners  are  sharp 
and  plainly  indicate  that  the  fragments  have  not  been  worked  over 
by  water.  As  Hitchcock  pointed  out,  these  "pebbles"  are  of  many 
different  sorts. 

The  great  majority  of  the  fragments  belong  to  the  schists.  A 
phyllite  composed  of  quartz  and  sericite  (occasionally  with  metam Or- 
phic biotite)  as  essentials,  and  of  graphitic  and  iron  ore  with  zircons 
as  accessories,  is  very  common.  It  is  the  slightly  metamorphosed 
equivalent  of  the  phyllite  in  the  eastern  half  of  the  Ascutney  area. 
The  rocks  of  the  contact  aureole  of  the  main  mountain  are  also  rep- 
resented by  manj^  fragments  that  are  still  more  altered  forms  of  the 
phyllites  than  the  type  just  mentioned.  Certain  of  the  dark,  fine- 
grained blocks  are  made  up  essentially  of  cordierite  rendered  turbid 
by  numerous  microlitic  inclusions,  probably  sillimanite.  The  gneissic 
fragments  are  usually  of  the  varieties  found  in  the  fundamental  for- 
mation at  the  foot  of  Little  Ascutney.  They  are  typical  biotite- 
gneisses,  often  garnetiferous  and  sometimes  charged  with  epidote. 
The  mica-schist  of  the  basal  crystallines  has  a  place  in  the  breccia  as 
a  biotite-quartz-schist  with  little  accessory  material.  An  ainphibo- 
lite,  identical  in  composition  with  that  described  from  the  locality  at 
the  foot  of  Crystal  Cascade,  is  likewise  x)resent  among  the  blocks. 

Quartz  occurs  as  large,  angular  jjieces  from  single  crystals,  as  com- 
pact quartzite,  and  as  a  chalcedonic  variety.  Granular  epidote  with 
some  quartz  forms  smaller  fragments  up  to  1  inch  (2.6  centimeters)  in 
diameter,  suggesting  the  equivalent  of  the  metamorphosed  limestone 
of  the  Ascutney  contact  zone.  Large  broken  crystals  of  orthoclase  are 
also  common  and  are  unquestionably  fragmental  in  the  same  sense  as 
the  cement.     So  far  as  known,  there  is  only  one  igneous  rock  among 


DALY.]  BIOTITE-GEANITE    STOCK.    .  79 

the  breccia  fragments,  and  it  is  one  of  the  least  abundant  kinds.  It 
is  a  tyjpical  granite-porplijny  with  an  ideal  development  of  idiomor- 
pliic  quartz  and  feldspar  phenocrysts.  The  only  dark-colored  con- 
stituent is  biotite,  the  lamellae  of  which  are  always  grouped  after  the 
manner  of  segregations  and  never  seem  to  form  true  phenocrysts. 
This  is  the  only  component  of  the  breccia  which  is  not  to  be  found  in 
crystalline  schists  surrounding  Ascutney.  The  breccia  cement  itself 
may  be  regarded  as  the  comminuted  remains  of  the  broken-up  schists. 
Each  of  these  great  horselike  inclusions  is  now  seen  to  have  the 
composition,  structure,  and  possible  field  relations  of  a  true  fault 
breccia.  The  most  satisfactory^  explanation  of  them  would  attribute 
them  to  the  disrupting  action  of  vigorous  and  long-continued  differ- 
ential movements  in  an  ancient  zone  of  dislocation.  That  zone  was 
perhaps  nearly  coincident  with  the  present  course  of  the  two  large 
dikes  in  which  the  horses  are  embedded.  The  faulting  gradually 
prepared  the  material  and  recemented  it  into  a  new,  tough,  solid  rock. 
Later,  the  invading  eruptives  carried  off  large  masses  of  it  as  they 
forced  their  way  along  the  old  zone  of  dislocation  and  consequent 
weakness.  The  average  specific  gravity  of  the  horses  is  about  2.79; 
that  of  the  sj^enite  porphj^ry,  2.(346.  It  is  reasonable  to  suppose  that 
the  immersed  blocks  of  breccia  sank  to  their  present  position  rather 
than  that  they  were  carried  up  from  below. 

BIOTITE-GRANITE  STOCK. 

The  second  of  the  stocks  which  go  to  make  up  the  main  mountain 
is  much  the  smaller  of  the  two.  The  total  area  is  about  1  square 
mile  (2.6  sq.  km.).  The  highly  irregular  contact  line  touches  both 
syenite  and  phyllites  (see  PI.  VII). 

That  this  rock  (an  alkaline  Ijiotite-granite)  must  be  referred  to  a 
period  of  intrusion  different  from  that  of  the  Main  stock  was  a  matter 
of  somewhat  prolonged  stud}^  in  the  field.  An  early  examination  of 
specimens  and  outcrops  indicated  that  on  the  southeast  side  of  the 
mountain  along  the  contact  with  the  schists  there  were  two  points 
where  the  igneous  rock  changed  from  a  quartzless  hornblende-bearing 
phase  to  a  highly  quartzose  phase,  devoid  of  all  dark-colored  essen- 
tials save  an  occasional  shred  or  plate  of  biotite.  This  change  was  in 
all  cases  sudden,  and  each  phase  held  its  character  for  a  long  distance 
from  their  common  contact  with  the  schists.  A  second  visit  to  the 
same  localities  explained  the  true  relations.  It  showed  that  the  gran- 
ite is  not  of  the  nature  of  an  acid  flow  streak  on  a  large  scale  in 
the  syenite,  but  that  the  former  was  intruded  after  the  syenite  had 
consolidated.  A  characteristic  endomorphic  zone  that  developed  in 
the  granite  will  be  described  below.  A  slight  amount  of  alteration 
in  the  syenite  itself  may  be  detected.  It  is  of  the  nature  of  the 
formation  of  a  secondary  granophyric  structure  among  the  feldspars 


80  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 

of  the  once  thoroughly  granular  syenite,  but  it  is  sufficient  to  empha- 
size here  the  granitic  apophyses  in  the  nordmarkite  as  a  proof  of  the 
younger  date  of  the  more  acid  rock. 

In  one  of  these  apophyses  about  6  inches  (15.2  centimeters)  in  width 
the  walls  of  syenite  must  have  been  already  solid  when  the  granite 
was  intruded,  since  out  from  them  have  been  developed  a  large  num- 
ber of  quartz  prisms  standing  squarely  on  the  walls  and  terminated 
with  the  usual  planes  at  the  free  ends,  which,  of  course,  point  toward 
the  middles  of  the  apophysis.  The  prisms  average  3  or  4  centimeters 
in  length  by  1  centimeter  in  thickness.  Other  large  crystals  similar 
to  these  sessile  ones  are  to  be  seen  completely  surrounded  by  the 
granitic  matrix;  they  are  doubly  terminated.  This  apophysis,  while 
thus  allied  to  the  pegmatites,  is  yet  believed  to  be  a  true  offshoot  of 
the  granite  because  of  the  identity  of  composition  existing  between 
its  matrix  and  the  material  of  the  more  normal  apophyses.  An  anal- 
ogy is  to  be  found  in  a  syenite  dike  cutting  the  classic  laurvikite, 
wherein  cryptoperthite  crystals  take  the  place  of  the  large  quartzes  of 
the  Ascutney  dike.* 

The  granite  has  been  mapped  with  a  fairly  close  degree  of  accuracy. 
The  very  dense  second-growth  timber  and  a  lack  of  outcrops  pre- 
vented the  discovery  of  the  contact  line  in  some  places. 

In  three  abandoned  quarries  situated  in  the  southern  lobe  of  the 
stock,  about  250  yards  (225  meters)  from  the  schist  contact,  a  more 
or  less  well- developed  master  jointage  is  to  be  seen.  Its  chief  interest 
consists  in  the  evident  independence  of  this  jointage  and  the  present 
topography  of  the  deep  valley  in  which  the  quarries  are  situated. 
The  latter  lie  on  an  east-west  line.  At  the  main  quarr}',  in  the  mid- 
dle, the  joint  planes  dip  about  15"  south,  or  a  few  degrees  east  of 
south.  At  the  eastern  and  western  quarries  they  are  found  to  swing 
into  an  easterly  and  westerlj;-  direction  of  dij)  respectively.  In  the 
western  quarry  the  ground  slopes  east  by  south;  in  the  eastern,  west 
by  south.  In  other  words,  the  jointage  is  quaquaversal  in  the  bottom 
of  a  steep- walled  ravine  or  gorge.  It  must  be  said  that  such  jointing 
can  hardly  be  explained  by  the  formation  of  concentric  rift-planes 
through  atmospheric  changes  of  temperature.  The  conditions  seem 
rather  to  confirm  the  more  general  view  that  the  structure  is  an  orig- 
inal phenomenon  due  to  cooling. 

The  granite,  excepting  in  a  narrow  contact  zone,  remains  quite  uni- 
form in  character  throughout  the  stock.  It  is  a  typical  pseudo- 
porphyritic  alkaline  biotite-granite  (spec.  2).  The  color  is  a  light 
grayish  X3ink,  the  grain  medium  to  coarse,  the  structure  liypidiomor- 
phic  granular  in  the  groundmass.  The  phenocryst-like  constituents 
are  quartz,  microperthite.  orthoclase,  and  soda-orthoclase  (with  a  spe- 
cific gravity  of  2.584  at  16"^  C),  accompanied  by  a  small  proportion  of 
biotite  individuals.     These  minerals  have  the  same  features  as  in 


a^rogger,  Zeit  fur  Kryst.,  Vol.  XVI,  1890,  p.  193- 


DALY.] 


BIOTITE-GBANITE    STOCK. 


81 


the  nordmarkites.  They  compose  the  groundmass  in  which  a  con- 
siderable amount  of  multiple-twinned  albite  (near  Abg  An,)  and 
titanite  with  some  magnetite,  zircon,  apatite  also  occur.  The  pheno- 
crysts  are  not  sharply  separated  from  the  groundmass  in  size,  and  it  is 
likely  that  there  has  been  only  one  generation  of  those  constituents. 
The  biotite  is  nearly  uniaxial.  The  large  amount  of  FeO  in  the  analy- 
sis suggests  that  it  is  a  lepidomelane.  Titanite  is  beautifully  crystal- 
lized with  the  ordinary  rhombic  outlines  and  well-developed  prismatic 
cleavage.     Its  pleochroism  is  strong: 

a,  pale  yellow. 

b,  yellow. 

c,  reddish  yellow. 
c>b>a. 

Pseudomorphs  of  magnetite  (ilmenite?)  after  titanite  are  not  uncom- 
mon. In  one  out  of  seven  slides  made  from  this  rock  a  single  small 
individual  of  pale-green  amphibole  was  discovered.  Augite  fails 
entirely.  Here,  as  in  the  other  stock  rocks  of  the  area,  the  quartz  is 
rich  in  liquid  and  gaseous  inclusions  and  in  negative  crystals  arranged 
in  lines  and  also  provided  with  double  bubbles  of  gas  immersed  in 
liquid. 

The  order  of  crystallization  is  the  normal  one: 

1.  Titanite,  apatite,  zircon,  and  magnetite. 

2.  Lepidomelane. 

3.  Albite  and  ortlioclase. 

4.  Microperthite. 

5.  Quartz. 

The  essential  oxides  (see  Table  X,  col.  1,  p.  84)  and  their  molecular 
proportions  are  noted  in  the  following  table: 


SiO^. 

AI2O3 

Fe^Oa 

FeO- 

MgO 

CaO. 

Na,0 

K2O- 

TiO., 

ZrO, 

P,0. 


Molecular 
proportions. 


1.1980 
0. 1382 
0. 0070 
0.0120 
0. 0082 
0. 0201 
0. 0723 
0.0511 
0. 0043 
0. 0003 
0. 0008 


If  all  the  Ti02  be  ascribed  to  titanite,  and  if  we  assume  that  the 
mica  contains  10  per  cent  MgO,  8  per  cent  KjO,  and  40  per  cent  SiOg, 
Bull.  209—03 6 


82  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 

the  analysis  may  be  calculated  and  the  quantitative  inineralogical 
composition  determined,  with  small  degree  of  error,  as  follows: 

Per  cent. 

Albite  molecule 37. 9 

Orthoclase  molecule 27. 0 

Quartz . 25.0 

Anorthite  molecule  _  _ . 3. 9 

Biotite  - .  - 3.8 

Magnetite . 1.6 

Titanite 0.9 

Apatite 0.3 

Zircon 0.1 

100. 0 

Chemically,  this  rock  is  an  ideal  equivalent,  among  the  alkaline 
rocks,  of  granite  among  the  nonalkaline  eruptives,  a  biotite-granite 
characterized  by  a  high  total  of  alkalies  with  the  soda  and  potash  in 
nearly  equal  proportion.  Iron,  lime,  and  magnesia  are  all  low.  It  is 
again  to  the  Christiania  region  that  we  must  go  for  the  already 
described  type  nearest  to  this  one.  The  "granitite"  of  Lier  affords 
an  analj^sis  which  is  noted  in  column  4,  Table  X.  In  the  Norwegian 
field,  as  at  Ascutnej^,  the  biotite-granite  is  the  youngest  eruptive 
excepting  the  lamprophyres.  A  close  and  interesting  correspondence 
between  these  two  rocks  is  further  illustrated  in  the  character  of  the 
endomorphic  contact  phase.  It  is  notably  granophyric  and  miarolitic 
in  the  Norwegian  occurrence,  and,  as  we  shall  see,  is  in  these  respects 
similar  to  the  Ascutney  rock. 

BASIC   SEGREGATIONS   IN   THE   GRANITE. 

The  homogeneity  of  the  biotite-granite  is  affected  by  the  presence 
of  nodular  basic  segregations  which,  while  not  nearly  so  abundant  as 
in  the  nordmarkites,  are  characteristic  of  the  rock.  They  vary  in 
color,  composition,  and  size.  Three  classes  may  be  distinguished,  not 
only  from  each  other,  but,  as  well,  from  the  metamorphosed  schist 
inclusions,  which  occasionally  appear  within  the  mass  of  the  stock. 

The  commonest  segregation  is  of  a  more  basic  character  than  the 
other  two  kinds  (spec.  la).  It  is  dark  greenish  gray  in  color,  spher- 
ical, oval,  or  lenticular  in  form,  and  in  the  hand  specimen  sharply 
outlined  against  its  host.  In  thin  section,  however,  it  is  once  more 
seen  that  this  macroscopically  sharp  outline  of  a  segregation  does  not 
forbid  a  very  intimate  interlocking  of  its  minei-als  with  those  of  the 
host.  The  size  may  vary  from  that  of  a  pea  to  nodules  as  large  as  a 
man's  fist.  Under  the  microscope  the  nodule  is  seen  to  be  a  panallo- 
triomorphic  aggregate  of  much  biotite,  hornblende,  and  triclinic  feld- 
spar always  close  to  and  averaging  the  oligoclase,  AbjAuj,  together 
with  smaller  amounts  of  microperthite  and  orthoclase.  Intei'stitial 
quartz,  much  titanite  and  apatite,  and  a  remarkably  small  amount  of 


DALY.]  BASIC    SEGREGATIONS    IN    THE    GRANITE.  88 

magnetite  comprise  tlie  accessories.     The  hornblende  is  not  identical 
with  that  of  the  syenite  nodules,  as  is  shown  by  the  j)leocln'oism : 

a,  pale  yellowish  green. 

b,  olive-green  (medium  to  strong  absorption). 

c,  olive-green  with  a  strong  bluish  cast  (medium  to  strong 
absorption). 

b>c>a. 

The  other  constituents  have  the  same  projjerties  as  the  parent  rock. 
Often  in  this  class  of  segregation  there  are  small  secondary  nodules 
of  nearly  pure  biotite  and  hornblende,  some  of  which,  by  the  concen- 
tration of  the  mica  around  the  j)eriphery,  recall  the  kernels  of  the 
nordmarkites.  On  the  other  hand,  the  general  continuity  of  the  main 
segregation  may  be  interruj^ted  by  light-colored  spots  composed  chiefly 
of  oligoclase,  quartz,  and  idiomorphic  hornblende. 

But  one  example  of  the  second  type  of  segregation  has  been  found. 
This  is  a  gray,  roundish  mass  about  7  feet  (2.1  meters)  in  diameter 
occurring  near  the  2,100-foot  contour  close  to  the  northern  contact 
(with  the  syenite)  of  the  great  southwestern  tongue  of  the  granite 
(spec.  113).  This  large  nodule  is  strongly  alkaline,  the  predominant 
feldspars  being  microperthite,  albite  (pure  or  charged  with  the  anor- 
thite  molecule  up  to  the  limit,  AbgAuj),  microcline,  and  probably 
cryptoperthite — named  in  the  order  of  their  abundance.  Free  quartz 
makes  up  probably  as  much  as  one-third  of  the  rock.  A  hornblende 
that  has  not  been  observed  in  any  other  rock  of  the  area  is  the 
remaining  essential.  It  forms  long,  narrow,  microlitic,  irregularly 
terminated  blades.     It  is  pleochroic  according  to  the  scheme : 

a,  light  greenish  yellow. 

b,  deep  brownish  green. 

c,  deep  brownish  green  with  a  strong  bluish  tinge. 
b=c>a. 

Much  idiomorphic  titanite,  a  few  rare  corroded  plates  of  biotite, 
many  crystals  of  magnetite,  considerable  apatite,  and  very  rare  zircons 
form  the  list  of  accessories.  The  structure  is  here  hypidiomorphic 
granular. 

Allied  to  the  second  type  is  a  third  class  of  the  segregations,  type 
analyses  of  which  are  given  (Table  X,  cols.  2  and  3,  p.  84).  Here  the 
biotite  is  much  more  common  than  the  hornblende,  the  feldspars  and 
accessories  remaining  the  same  in  nature  and  relative  abundance 
(spec.  lb).  Quartz  is  not  so  abundant.  The  structure  is  the  hypidio- 
morphic granular.  The  feldspars  are  somewhat  altered,  as  is  indi- 
cated in  the  chemical  analysis.  Calcite  and  a  little  muscovite  are  the 
secondary  products.  The  microscopic  diagnosis  and  the  analysis 
agree  in  putting  these  segregations  among  the  alkaline  quartz  syenites 
(akerites).  Again,  it  will  be  observed  that  there  is  an  especially  large 
amount  of  the  mineralizer,  fluorine,  in  the  segregation. 


84  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VEEMONT.         [bull.  209. 

Table  X — Analysis  of  biotite-granite. 


3. 


SiO, 

AI2O3 

FeA 

FeO 

MgO 

CaO 

Na^O 

K2O 

H2O  above  110°  C. 
H2O  below  110°  C. 

CO2 

TiO^ 

ZrO, 

PA 

CL_.. J 

F 

FeS, 

NiO,  CoO 

MnO 

BaO 

SrO 

LLO 


0=F,CL 

Total  S-  - 
Sp.  gr... 


71.90 
14.12 
1.20 
0.86 
0.33 
1.13 
4.52 
4.81 
0.42 
0.18 
0.21 
0.35 
0.04 
0.11 
0.02 
0.06 
Trace. 


0.05 
0.04 

Trace. 

Trace. 


59.27 
15.76 
2.07 
8.57 
3.04 
3.69 
5.63 
3.33 
0.74 
0.23 
0.30 
1.12 
0.04 
0.42 
0.03 
0.42 
0.07 

Trace. 
0.37 

Trace? 
Faint  tr. 

Trace. 


56.01 
8  15.19 
2.34 
4.89 
4.67 
4.85 
5.66 
2.16 
0.36 
0.90 
Undet. 
1.13 


100. 85 
0.03 


100. 32 
Trace. 
2.616 


100.10 
0.19 


0.53 

Undet. 

Undet. 

0.09 

0.03 

0.40 

Trace? 


99.91 
0.037 
2.661 


99.21 


2.720 


75. 74 
13.71 

0.55 

Trace. 
1.26 
8.72 
4.69 

0.46 


0.17 


100. 30 


ft  "With  ZrOo. 

1.  Biotite-granite,  Asciitneyville  quarries;  analysis  by  Hillebrand. 

2.  Basic  segregation  in  the  biotite-granite:  analysis  by  Hillebrand. 

3.  Another  sample  of  the  last,  containing  more  hornblende;  analysis  by  Hille- 
brand. 

4.  '"Granitite"  from  Lier,  Christiania  region;  Brogger,  Zeit.  ftir  Kryst.,  Vol. 
XVI,  1890,  p.  72. 

ENDOMORPHIC  ZONE  OP  THE  GRANITE. 

The  detection  in  the  field  of  the  actual  plane  of  contact  of  granite 
and  syenite  is  rendered  comparatively  easy  on  account  of  a  con- 
spicuous structural  variation  whicli  characterizes  the  endoniorphic 
zone.     At  the  average  distance  of  20  feet  (6.1  meters)  from  the  con- 


DALY.]  LAMPEOPHYEES.  85 

tact  the  normal  granite  becomes  much  more  pori)hyritic  in  appear- 
ance (spec.  105).  The  phenocrysts  are  chiefly  quartz,  which  may 
either  retain  its  dimensions  in  the  normal  rock  or  may  form  much 
larger  doubly  terminated  crystals.  The  groundmass  is  much  finer 
grained,  though  always  holocrystalline,  and  is  either  granophyric  or 
identical  in  general  structure  with  the  normal  granite.  Large  ter- 
minated crystals  of  quartz  may  sometimes  be  seen  projecting  from 
the  syenite  into  the  granite  in  the  same  way  as  in  the  apophyses 
already  described.  Especially  marked  on  the  ledges  of  the  contact 
zone  are  abundant  roundish  miaroles  from  1  inch  (2.6  centimeters) 
to  3  inches  (7.8  centimeters)  in  diameter,  either  completely  filled  with 
crystalline  matter  or  presenting  cavities  lined  with  well-formed 
crystals  (spec.  106). 

The  usual  occupants  of  the  miaroles  are  quartz  crystals  showing 
the  common  terminal  and  prismatic  i3lanes  and  crystals  of  the  same 
feldspars  as  occur  in  the  granite  proper.  The  feldspars  are  flesh  col- 
ored or  light  brownish  and,  in  thin  section,  turbid.  They  bear  the 
planes  (001)  (010)  (110)  (101)  (201)  (021)  and  (111);  (010)  and  (001) 
are  especially  wall  developed.  The  commonest  of  these  feldspars 
seems  to  be  a  genuine  microperthite.  It  illustrates  in  excellent 
fashion  the  rare  Manebacher  law  of  twinning  and  the  murchisonite 
cleavage.  The  triclinic  feldspar  of  the  intergrowth  is  pure  albite, 
giving  an  extinction  angle  of  19°  on  (010).  Microcline  and  orthoclase 
crystals  also  occur  in  the  miaroles.  The  latter  has  the  small  optical 
angle  of  sanidine.  Finally,  pseudomorphs  of  limonite  after  siderite 
completes  the  list  of  the  minerals  which  have  been  found  in  the  mia- 
roles. Biotite  seems  to  be  absent.  A  close  parallel  to  this  endo- 
morphic  zone  is  furnished  in  the  aplitic  granophyre  described  by 
Brogger  as  the  contact  phase  of  the  alkaline  biotite-granite  of  Lier,<^ 
which  in  other  respects  resembles  the  Ascutnej^  rock. 

The  endomorpliic  zone  has  been  enriched  by  the  incorporation  of  a 
certain  amount  of  basic  material  evidently-derived  from  the  syenite. 
In  the  granophyric  groundmass  there  are  sporadic  irregular  granular 
areas  impregnated  with  an  alkaline  hornblende  near  barkevikite  and 
biotite.  These  are  not  found  where  the  granite  is  in  contact  with  the 
phyllites. 

LAMPROPHYRES. 

A  number  of  dikes  of  lamprophyric  habit  cut  the  syenites  at  various 
points,  and  rocks  of  the  same  character  intersect  the  Basic  stock  and 
the  schists.  No  such  dike  has  been  discovered  in  the  granite  stoclc, 
but  it  is  probable  that  they  are  all  younger  than  that  stock.  They 
belong  either  to  the  class  of  camptonites  or  to  the  class  of  diabases. 
A  similar  association  of  the  two  groups  in  the  same  region  has  been 
described  by  Kemp  as  occurring  on  the  Maine  coast,* 

aZeit.  fiii-  Kryst.,  Vol.  XVI,  1890,  p.  7■^.  6Bull.  Geol.  Soc.  Am.,  Vol.  I,  1890,  p.  32. 


86  GEOLOGY    OF    ASCUTNEY   MOUNTAIN,   VEEMONT.         [bull.  209. 

CAMPTONITES. 

At  the  top  of  Little  Ascutney  a  camptonitic  dike  cuts  the  nord- 
markite-porphyry  dike,  the  paisanite  dike,  a  horse  of  the  breccia, 
and  the  diorite.  The  rock  is  a  very  compact  grayish-black  mass,  in 
which  here  and  there  a  hornblende  crystal  and,  more  rarely,  a  feld- 
spar appear  as  phenoerysts  (spec.  57).  In  thin  section  it  is  seen  to  be 
an  acid  camjDtonite  of  the  usual  structure  and  composition.  The 
hornblende  phenoerysts  are  idiomorphic  and  measure  from  1  to  3 
millimeters  in  length.  The  pleochroism  and  absorption  are  those  of 
a  common  basaltic  hornblende: 

a,  pale  brownish  yellow. 

b,  deep,  rich  brown. 

c,  deep,  rich  brown. 
c>b>a 

The  extinction  on  (010)  is  about  15°  30'. 

The  plagioclase  is  apparently  very  uniform  in  composition  and 
averages  the  basic  labradorite,  AbgAng,  both  in  the  rare  phenoerysts 
and  in  the  groundmass. 

The  rock  is  greatly  altered,  and  this  characteristic  adheres  to  all 
the  camptonites  of  the  area.  Chlorite,  epidote,  calcite,  secondary 
quartz,  and  kaolin  are  the  products  of  decomposition. 

Analysis  1,  in  Table  XI,  represents  the  approximate  composition  of 
the  average  camptonite  of  the  area  (spec.  74).  It  was  made  from  a 
type  differing  from  that  just  described  in  containing  a  small  i^ropor- 
tion  of  augite  in  the  groundmass  and,  more  rarely,  among  the  pheno- 
erysts. The  feldspar  is  here  again  the  labradorite  Ab^Aug.  The 
augite  is  much  altered  (into  uralitic  amphibole,  chlorite,  and  the 
ores)  from  its  original  diopsidic  condition.  Dikes  corresponding  to 
this  analysis  were  found  cutting  the  syenite  on  the  Windsor  trail 
about  500  yards  (457  meters)  from  the  main  summit  of  Ascutney 
Mountain,  cutting  coarse  diorite  in  the  saddle  at  the  east  end  of 
Little  Ascutney,  and  cutting  gneiss  east  of  the  notch  road  and  south- 
west of  Brownsville. 

While  there  is  a  noteworthy  difference  between  this  analysis  and 
that  of  Hawes's  classic  camptonite,  the  former  agrees  well  with  the 
average  analysis  of  camptonite  as  calculated  by  Brogger  (cf.  Table 
XL) 


DALY.] 


OAMPTONITE. 

Table  XI. — Analysis  of  camptonite. 


87 


1. 

2. 

3. 

SiO^ 

48.22 

14.27 
2.46 
9.00 
6.24 
8.45 
2.90 
1.93 
1.66 
0.28 
0.15 
2.79 
0.03 
0.64 
0.10 
0.05 
0.36 
0.03 
0.20 
0.04 

Trace. 

Trace. 

Trace. 

41.94 
15.36 
3.27 
9.89 
5.01 
9.47 
5.15 
0.19 

}        3.29 

2.47 
4.15 

43.65 

AI2O3 .  . 

16  29 

FeA 

FeO 

14  76 

MgO... 

5  96 

CaO 

10  16 

NaaO 

3  05 

K2O 

1  50 

H2O  above  110°  C . 

H2O  below  110°  C 

CO2 

TiO^ 

4  63 

ZrO^ 

P2O5 

CI 

F. 

FeSj 

NiO,CoO . 

MnO 

0. 25 

BaO . 

SrO 

LijO 

CtiO 

0=F,C1 

99.80 
0.04 

100.44 

100.00 

99.76 

0.19 

2. 810-2. 869 

\ 

Totals.   -     - 

Sp.gr 

1.  Camptonite  dike,  Ascutney  Mountain;  analysis  by  Hillebrand. 

2.  Classic  camptonite,  Campton  Falls,  N.  H.;  Rosenbnsch:  Elem.  der  Gesteins- 
lehre,  2ded.,  1901,  p.  244. 

3.  Average  analysis  of  eight  camptonite  dikes;  Brogger:  Quart.  Jour.  Geol. 
Soc,  Vol.  L.  1894,  p.  26. 

The  hornblende  has  not  been  analyzed;  it  is,  hence,  not  possible  to 
calculate  the  analj^sis.  The  specific  gravity  of  these  dikes  varies 
from  2.810  to  2.869;  on  account  of  alteration,  no  two  pieces,  even 
from  the  same  hand  specimen,  will  agree  in  specific  gravity. 

DIABASE  DIKES. 

The  second  class  of  melanocratic  dikes  comprises  compact,  equi- 
granular  or  porphyritic  diabases  of  normal  composition  and  structure, 


88  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull. 209. 

thus  not  differing  essentially  from  the  common  dikes  of  the  same  rock 
occurring  so  abundantly  up  and  down  the  Connecticut  Valley 
(spec.  120).  The  feldspar  is  here  also  near  the  basic  labradorite 
AbgAug.  Like  the  pale-green  intersertal  augite,  it  is  much  affected 
by  weathering ;  the  products  of  the  change  are  the  same  as  in  the 
camptonites.  A  sulphide,  probably  pyrite,  is  visible  in  notable 
amount,  even  in  the  hand  specimen.  The  magnetite  is  strongly  titan- 
iferous.  Zircon  and  titanite  are  absent,  and  there  is  comparatively 
little  apatite.  The  specific  gravity  was  measured  at  2.922.  The 
total  analj^sis  is  given  in  Table  XII.  It  corresponds  to  that  of  a 
common,  somewhat  weathered  diabase. 

Table  XII. — Analysis  of  diabase  (by  Hillebrand) 

Si02._--. .-  49.63 

AI2O3 . 14.40 

FeA . 2.85 

FeO 8.06 

MgO 7.25 

CaO 9.28 

Na^O 2.47 

K26 0.70 

H3O  above  110°  C 1.47 

H2O  below  110°  C 0.27 

CO2 1.36 

TiO^ 1.68 

ZrOa - Trace? 

PA 0.25 

CI 0.07 

F Trace 

FeS., 0.22 

NiO,CoO 0.04 

MnO - 0.17 

BaO Trace? 

SrO Trace? 

LiaO Trace 

100. 17 
0=F,  CI 0.02 

100. 15 

Totals 0.12 

Sp.gr 2.922 

SUMINIARY. 

The  list  of  eruptive  rocks  in  Mount  Ascutney  includes  the  Basic 
stock  of  gabbros  and  diorites  transitional  into  one  another  and  into 
an  acid  essexitic  phase,  ramifying  dikes  of  younger  diorite,  dikes  of 
a  rock  type  not  heretofore  described  and  called  "wiudsorite"  from 
this  Ascutney  occurrence,  the  nordmarkite-porphyry  dike-like  stock 
of  Little  Ascutney,  the  pulaskite  stock  of  Pierson  Peak,  the  great 
paisanite  dike  of  Little  Ascutney,  the  variable  nordmarkites  of  the 
Main  syenite  stock  with  granitic  and  monzonitic  phases,  the  homo- 
geneous alkaline  biotite  granite  of  the  Ascutneyville  stock,  and  the 


DALY.]  GENERAL    FEATURES    OF    THE    ERUPTIVES.  89 

aplites  and  lamproph^yres  (diabase  and  cami3tonites)  cutting  nearly 
all  the  other  bodies. 

Chemically,  the  series  of  eriiptives  as  a  whole  is  characterized  by 
normal  silica,  high  alkalies,  the  potash  slightly  predominating,  nor- 
mal alumina,  medium  iron,  low  magnesia,  low  lime,  and  high  titanic 
oxide.  Mineralogically,  they  are  rich  in  feldspar  which  is  generally 
microiperthitic,  and  are  poor  in  biotite  and  bisilicates.  Especially 
noteworthj^  is  the  extraordinary  development  of  indigenous  basic 
nodules  of  segregations  and  of  ' '  schlieren  "  in  the  different  rock  bodies, 
including  both  dikes  and  stocks.  The  abnormal  abundance  of  the 
segregations  is  i)robably  to  be  connected  with  the  smallness  of  the 
conduit  through  which  each  irruption  took  place.  Variations  of  tem- 
perature, abundance  of  foreign  fragments,  and  the  repetition  of  intru- 
sion in  the  same  conduit  have  all  played  their  part  in  disturbing  the 
normal  process  of  crystallization. 

Considering  the  small  area  occupied  by  the  Ascutney  intrusives, 
they  must  be  considered  as  having  an  unusually  wide  range  of  com- 
position (see  list  on  p.  36).  While  dioritic  types  fail  in  the  allied 
petrographical  iDrovince  of  the  Christiania  region  and  nordmarkites 
and  related  alkaline  rocks  are  absent  in  the  Monzoni  region,  both  of 
these  classes  are  represented  at  Ascutney.  The  intimate  association 
of  independent  bodies  of  such  nonalkaline  rocks  as  the  gabbros  and 
diorites  of  the  oldest  stock  and  of  the  older  basic  dikes,  with  the 
several  bodies  of  typical  alkaline  syenites  and  granite,  is  to  be  par- 
tieularlj^  emphasized.  That  such  an  association  is  not  rare,  in  America 
at  least,  is  shown  by  its  repeated  occurrence  in  New  York  State '^  and 
in  Essex  County,  Mass.^  Not  the  least  significant  fact  concerning  the 
Ascutney  eruptive  group  is  the  occurrence  of  rock  types  transitional 
between  the  nonalkaline  and  alkaline  irruptives.  Thus  there  is  not 
only  the  most  striking  consanguinity  among  the  respective  members 
of  each  of  these  classes,  but  the  two  classes  are  themselves  allied  by  a 
family  relationship  which  is  reflected  also  in  many  details  of  miner- 
alogical  and  chemical  composition. 

Among  the  details  described  in  connection  with  the  irruptives,  we 
may  recall  some  which,  while  of  greater  or  less  importance  to  the  gen- 
eral geology  or  mineralog}^  of  the  area,  are  not  implied  in  the  forego- 
ing resume  of  the  intrusions.  These  include  the  evidence  for  the 
cylindrical  character  of  the  main  controlling  stock  of  Mount  Ascutney 
itself,  the  remarkable  tarnishing  which  slight  exposure  produces  in 
the  nordmarkite  of  the  Windsor  quarry  and  in  the  related  paisanite 
of  Little  Ascutney,  the  great  masses  of  breccia  in  the  nordmarkite 
porphyry  and  paisanite  dikes,  and  the  interesting  endomorphic  phases, 
especially  of  the  biotite-granite.  Finally,  the  considerations  relative 
to  the  mode  in  which  the  conduit  has  become  occupied  by  the  different 
intrusives  will  be  summarized  in  the  following  chapter. 

aCushing,  Bull.  Geol.  Soc.  Am.  Vol  X,  1899,  p.  177.    Smyth,  Ibid.,  Vol.  VI,  1895.  p  263.    Eakle, 
Am.  Geol.,Vol.  Xll,  1893,  p.  35. 
6 Washington,  Jour.  Geol.,  Vol.  VI,  1898  p.  799. 


CHAPTER  V. 

THEORETICAL   CONCLUSIONS. 
MA:N^]SrER  OF  IXTRUSIOK  OF  THE  STOCKS. 

Probably  the  most  important  question  in  connection  with  the  dy- 
namical geologj''  of  the  mountain  centers  about  the  actual  method  of 
injection  followed  by  each  of  the  five  largest  eruptive  bodies. 

Hitherto  it  has  been  assumed  that  the  Ascutney  rocks  belong  to  the 
category  of  genuine  intrusives  and  that,  for  example,  we  have  not 
here  to  do  with  a  deeply  eroded  volcanic  neck  or  pipe.  This  view 
will  be  still  held  in  the  further  discussion  of  the  rock  bodies.  The 
evidence,  however,  that  the  granitic  rocks  now  exj)osed  do  not,  after 
all,  represent  the  deep-seated  equivalents  of  magmas  which,  in  the 
eruptive  periods,  escaped  at  the  earth's  surface  as  lava  flows,  is  largelj^ 
but  negative.  To  exclude  this  volcanic  hypothesis  it  is  clearly  not 
sufficient  that  the  igneous  bodies  have  all  the  characteristics  of  a  plu- 
tonie  origin  and  that  there  is  in  the  vicinity,  at  present,  lack  of  explo- 
sion breccias  or  lavas  in  any  form.  The  complete  removal  of  such 
products  is  only  a  question  of  the  time  allowed  and  the  depth  of 
denudation  since  the  early  period  of  the  eruptions.  Nor,  again,  and 
just  as  surely,  is  the  general  lack  of  flow  structure  showing  a  rela- 
tively rapid  upward  movement  of  the  magma  of  distinct  help  in  decid- 
ing the  alternative. 

The  ]3eculiar  arrangement  and  form  of  the  stocks,  especially  the, 
lobate  plan  of  the  granite,  seem  to  give  greater  satisfaction  intheplu- 
tonic  hypothesis.  But  a  much  stronger  reason,  affording  cumulative 
evidence  for  its  adojjtion,  is  doubtless  to  be  found  in  the  analogy  of 
Ascutney  Mountain  with  a  score  of  other  granitic  areas  in  Vermont 
alone.  There  appears  to  be  no  evidence  at  all  for  the  volcanic  nature 
of  these  igneous  bodies.  Although  the  geological  conditions  are  often, 
particularly  in  the  smaller  areas,  very  similar  to  those  at  Ascutney, 
extrusive  rocks  seem  never  to  be  organically  associated  with  a  single 
granitic  mass.  So  far  as  known,  there  is  no  more  reason  to  attribute 
a  volcanic  origin  to  any  one  of  these  smaller  granitic  bodies  in  Ver- 
mont than  to  the  others;  nor,  indeed,  than  to  the  great  massifs  typi- 
fied by  the  Barre  granite.  The  transition  in  size  from  the  smallest  to 
the  largest  area  is  gradual,  and  mere  size  and  shape  will  not  suffice  as 
a  criterion  of  volcanic  origin  for  any  one.  It  is,  perhaps,  not  too  much 
to  say  that,  if  the  Ascutney  bodies  occupy  the  vent  of  an  ancient 
greatly  eroded  volcanic  neck,  the  Barre  granite  itself,  contrary  to  the 
90 


DALY.]  THEOEIES    APPLICABLE    TO    THE    INTEUSIONS.  91 

received  opinion  of  geologists,  may  be  regarded  as  possibly  of  the 
same  origin.  In  other  words,  it  is  reasonable  to  believe  that  the  vol- 
canic hypothesis  for  Ascutney  Mountain  has  no  stronger  foundation 
in  fact  than  it  has  for  normal  granitic  areas  the  world  over.  In  this 
view,  the  igneous  rocks  of  Ascutney  are  all  Irruptive  and  each  irrup- 
tive  body  assumed  its  present  position  and  full  volume  only  after 
some  process  had  prepared  the  corresponding  space  within  the  coun- 
trj^  rock^.  The  problem  before  us  relates  to  the  manner  in  which  that 
preparation  was  carried  out. 

APPLICATION    OF    EXISTING    THEORIES    TO    THE    ASCUTNEY 

INTRUSIONS. 

It  is  held  by  some  of  our  ablest  masters  in  petrological  science — 
perhaps  by  most  of  them — that  stocks,  sills,  laccoliths,  and  dikes  are, 
from  the  point  of  view  of  dynamical  geology,  of  the  same  nature — i.  e., 
that  they  vary  only  in  size  and  form.  Each  is  composed  of  a  con- 
solidated rock  magma  which  has  been  injected  into  the  country  rock 
because  of  a  previous  or  accompanying  oj)ening  of  cavities  within  the 
earth's  crust.  Displacements  of  folded  and  faulted  rocks  in  mountain 
massifs  are  made  responsible  for  the  cavities  or  chambers.  The  lat- 
ter may  be  supposed  to  antedate  the  eruption  (plainly  an  inadmissible 
premise  for  the  larger  eruptive  bodies)  or  to  have  been  magmatically 
filled  pari  passu  with  the  dislocation.  In  either  case  the  adherents 
of  this  theory  believe  that  no  important  assimilation  of  the  country 
rock  by  the  invading  magma  takes  jjlace,  and  that,  therefore,  the 
composition  of  the  magma  is  not  affected  by  such  assimilation. 

It  is  clear  from  the  foregoing  discussion  of  the  Ascutney  intrusives, 
as  well  as  from  the  inspection  of  the  map  (PI.  VII),  that  none  of  the 
larger  ones  is  laccolithic  in  character.  The  syenite-porphyry  of  Little 
Ascutney  has  apparently  followed  a  zone  of  weakness,  the  contact  of 
the  gneiss  and  Basic  stock.  Though  for  this  reason  dike-like  in  its 
geological  relations,  it  has  so  far  enlarged  its  conduit  as  to  assume 
the  proportions  of  a  stock.  The  other  four  bodies  are  true  stocks. 
Each  eruptive  cuts  across  the  schists,  so  that  the  igneous  contact  gen- 
erally stands  at  a  high  angle  to  the  strike  of  the  schists.  Only  at  the 
eastern  end  of  the  mountain  are  the  two  parallel,  and  it  has  already 
been  noted  that  the  surface  of  contact  of  the  Main  syenite  stands 
nearly  vertical,  and  is  thus  not  coincident  with  the  dip  plane  of  the 
schists  there  any  more  than  on  the  other  slopes  of  the  mountain. 
The  facts  show  that  a  laccolithic  origin  can  not  be  hypothecated  for 
the  Main  syenite,  much  less  for  the  granite. 

The  east- west  elongation  of  the  igneous  area  as  a  whole  might  sug- 
gest that  the  intrusions  occupy  a  zone  of  dip  faulting  in  the  schists. 
But  the  transitional  contact  belt  of  the  phyllitic  and  gneissic  series  is 
easily  recognized  on  both  the  north  and  the  south  side  of  the  moun- 


"The  term  "country  rock"  is  used  in  this  report  as  a  convenient  expression  denoting  the  ter- 
rane  invaded  by  and  thus  in  contact  with  an  irruptive  body. 


92  GEOLOGY    OF    ASCUTNEY    MOUlSfTAIN,    VERMONT.         (bull.  209. 

tain,  and  it  is  quite  as  easily  seen  in  the  field  that  the  belt  has  not 
been  appreciably  offset  by  a  fault  transverse  to  the  strike.  It  is  like- 
wise in  the  highest  degree  improbable  that  displacements  iiarallel  to 
the  strike  of  the  schists  can  be  safely  called  upon  to  explain  the  spaces 
now  filled  with  the  solidified  magmas. 

The  facts  derived  from  field  study  also  speak  strongly  against  the 
idea  that  all  or  any  of  these  intrusions  took  jDlace  in  consequence  of 
the  removal  of  the  country  rock  en  bloc  bj^  faulting.  It  might  be 
conceived  that  faulting  could  have  led  to  the  transfer  of  the  displaced 
schists  upward,  as  if  punched  out  by  a  huge  die,^'  or  to  the  founder- 
ing of  the  corresponding  blocks,  which  would  thus  become  buried 
deeply  by  the  magma  entering  the  resulting  chamber.  If  such  circling 
faults  had  occurred  we  should  expect  some  evidence  of  them  to  be  yet 
decipherable  in  the  country  rocks.  Yet  the  latter  show  no  sign  of 
disturbance  that  can  be  traced  to  those  particular  movements.  Even 
if  the  intrusion  had  taken  place  after  this  manner  in  one  instance,  it 
is  in  the  highest  degree  improbable  that  so  unusual  a  dynamic  proc- 
ess should  have  been  repeated  three  times  in  this  limited  area.  The 
ground  plan  of  the  different  stocks  as  expressed  in  the  geological  map 
can  scarcely  be  explained  on  the  hypothesis  of  circling  faults.  We 
must  rather  conclude,  from  a  survey  of  the  ground,  that  each  of  the 
stock  bodies  has  actively  displaced  its  country  rock  so  as  to  find  room 
for  itself.  The  Basic  stock  has  displaced  the  gneisses;  the  nordmark- 
ite  stock  has  displaced  diorites  and  schists;  the  stock  of  Pierson 
Peak  has  displaced  gabbros  and  diorites;  the  nordmarkite-poriDhyry 
of  Little  Ascutneyhas  displaced  gneisses  and  diorites,  and  the  gran- 
ite has  displaced  the  syenite  and  a  small  amount  of  the  phyllites. 

A  second  commonly  held  view  of  many  stocks  and  of  many  "batho- 
liths"  is  that  they  have  undergone  their  "mise  en  place"  as  a  result 
chiefly  of  the  caustic  and  assimilating  property  of  the  igneous  magma 
in  contact  with  the  country  rock.  Thus,  while  Brogger  regards  the 
Predazzo-Monzoni  area  as  illustrating  in  a  thoroughgoing  manner  the 
process  of  differentiation  in  deep-seated  chambers  prepared  by  crustal 
movements,  Fouque  finds  in  the  same  province  an  almost  typical 
example  of  assimilation.  It  is  here,  as  elsewhere,  a  question  of  the 
degree  to  which  the  process  produces  its  effect,  as  every  field  geologist 
is  bound  to  credit  the  assimilation  of  small  foreign  fragments  caught 
in  a  molten  magma  and,  as  well,  the  local  and  subordinate  digestion  of 
the  walls  at  certain  of  the  eruptive  contacts  already  described  in  geo- 
logic literature. 

Stated  in  its  usual  form,  the  assimilation  hypothesis  also  will  hardly 
fit  the  facts  recorded  for  the  Ascutney  eruptives.  The  oldest  stock  is 
quite  basic,  though  it  cuts  a  series  of  acid  gneisses.  The  Main  syenite 
shows  no  basification  at  its  contact  with  the  diorites.  Its  endomorphic 
contact  phase  is,  on  the  contrary,  there,  as  elsewhere,  more  stronglj^^ 
quartzose  and  has  often  even  a  smaller  proportion  of  bisilicate  than 

"Recalling  the  "bysmalith"  of  Iddings,  Mon.  U.  S.  Geol.  Survey,  Vol.  XXXII,  Pt.II,1899,p.l6. 


DALY]  SUGGESTED    HYPOTHESIS    AS    TO    THE    INTEUSION.  93 

the  average  syenite.  The  biotite-granite  has,  it  is  true,  a  hornblende 
among  the  constituents  of  its  basic  segregations;  but  the  endomorphic 
zone,  where  the  granite  comes  in  contact  with  the  older  syenite,  does 
not  exhibit  any  special  sympathy  with  that  rock.  Still  more  impor- 
tant is  the  complete  lack  of  basification  in  the  pulaskite  of  Pierson 
Peak  which  cuts  the  diorite  and  associated  gabbro.  That  stock  is  of 
small  dimensions.  It  is  alkaline,  and  hence  is  composed  of  material 
which  in  its  magmatic  form  must  have  been  of  specially  caustic  nature. 
The  crystalline  rock  through  which  the  magma  found  its  way  has  a 
markedly  different  composition.  There,  if  anywhere,  we  should,  on 
the  assimilation  hypothesis,  look  for  an  endomorphic  zone  sensibly 
affected  by  the  country  rock.  The  failure  of  such  a  zone  is  as  unques- 
tionable as  in  the  notable  case  of  the  shonkinite  laccolith  of  Square 
Butte,  Montana.^'  The  Pierson  Peak  stock  is  made  up  of  a  homogeneous 
syenite  which  is  indistinguishable,  save  for  the  absence  of  free  quartz 
and  the  disappearance  of  much  of  the  essential  bisilicate,  from  an 
abundant  phase  of  the  Main  stock  of  Ascutney  Mountain  proper. 
Both  stocks  are  syngenetic  with  the  stock-like  dike  of  Little  Ascutney. 
The  occurrence  of  all  three  with  essentially  similar  mineralogical  and 
chemical  properties,  but  with  essentially  diverse  country  rocks,  seems 
to  prove  that  they  came  from  a  single  magma  which  persisted  in 
nearly  its  pure  form  even  after  injection  and  notwithstanding  the 
well-known  solvent  power  of  alkaline  magma  on  both  acid  and  iron- 
rich  basic  rocks. 

SUGGESTED  HYPOTHESIS  OF  THE  MANNER  OF  INTRUSION. 

Without  considering  other  and  less  important  views  of  the  mechan- 
ics of  intrusion,  which,  suggestive  as  they  are,  must  yet  be  regarded 
as  insufficiently  supported  by  observations  in  nature,  a  somewhat 
detailed  statement  may  be  made  of  a  third  hypothesis  which  has 
forced  itself  upon  the  writer.  It  not  only  explains  the  facts  as 
far  as  Mount  Ascutney  is  concerned,  but  meets  as  well  all  the  tests 
which  have  yet  been  ai^iDlied  to  it  from  the  results  of  experimental 
geology,  from  observations  in  other  regions,  and  from  the  theory  of 
igneous  bodies  generally. 

Most  geologists  are  agreed  that  intrusion  on  a  large  scale  is  not  a 
sudden  act,  but  occupies  a  period  of  time  comparable  to  that  required 
for  complex  folding  in  a  mountain  massif.  This  conclusion  has  been 
reached  by  those  advocating  the  assimilation  theory,  as  well  as  by 
those  holding  the  rival  theory  of  laccolithic  and  allied  crustal  dis- 
placement.^ While  the  conclusion  of  any  investigator  as  to  the  time 
required  for  a  magmatic  injection  is  itself  in  part  a  by-product  of  the 
intrusion  theory  ruling  in  his  mind,  it  is  yet  noteworthy  that  the 
present  exponents  of  the  opposed  theories  accord  in  ascribing  great 
duration  to  the  time  required  for  granitic  intrusions  at  least.     It  will, 


"Weed  and  Pirsson,  Bull.  Geol.  Soc.  Am.,  Vol.  VI,  1895,  p.  389. 

bW.  C.  Brogger,  Die  Eruptlvgesteine  des  Kristianiagebietes,  Vol,  II,  1895,  p.  149. 


94  GEOLOGY    OF    ASOUTNEY   MOUNTAIN,   VERMONT.         [bull.  209. 

then,  be  no  antecedent  objection  to  the  hypothesis  now  to  be  proposed 
that  it  is  based  on  a  process  of  integration  of  small  effects,  and  that 
the  integration  suffices  for  the  geological  work  in  hand  only  after  the 
lapse  of  a  long  period  between  the  beginning  and  end  of  the  process. 

The  starting  point  of  the  hypothesis  is  found  in  the  consideration 
of  the  phenomena  of  the  contact  belt  belonging  to  each  irruptive  body. 
The  present  assimilation  theory  of  Michel  Levy  and  others  demands 
a  study  of  the  same  belt  as  a  test,  rather  than  as  the  basis  of  formu- 
lation. 

At  many  points  in  the  internal  contact  zone  of  any  one  of  the  stocks 
numerous  fragments  of  the  countr}^  rock  may  be  seen  in  the  erup- 
tive (PI.  VI).  These  are  completely  isolated,  immersed  in  the  crys- 
tallized magma,  though  often  they  are  seen  to  have  moved  only  a  few 
feet  or  inches  from  the  parent  rock.  As  found  in  endomorphic  zones 
of  plutonics  generally,  the  fragments  are  further  quite  normal  in 
showing  angular  outlines  and  very  sharp  boundaries  against  the 
eruptive  rock.  There  is  usually,  indeed,  plain  indication  that  these 
fragments  have  suffered  little,  if  any,  chemical  solution  by  the 
naagma.  Recent  experiments,  however,  have  established  beyond  per- 
adventure  that,  at  temperatures  but  slightly  above  that  of  complete 
fusion  of  any  silicate  mixture,  every  important  rock-forming  mineral 
may  be  completely  dissolved  in  that  magma.*  The  conclusion  seems 
unavoidable  that,  at  the  moment  when  a  given  foreign  fragment  was 
torn  or  floated  off  from  its  wall  and  thereafter,  the  immersing  magma 
was  relativel^^  cool,  and  thus  enfeebled  in  its  solvent  power.  That  its 
metamorphosing  power  was  likewise  diminished  is  suggested  by  the 
fact,  borne  out  by  microscopic  study,  that  the  recrystallization  of  the 
fragment  is  generally  no  more  advanced  than  that  of  the  country 
rock  many  feet  from  the  irruptive. 

But  a  still  stronger  proof  of  a  comparatively  low  temperature  at  the 
moment  of  isolation  of  any  one  of  the  fragments  is  the  fact  that  it  is 
now  to  be  seen  floating,  as  it  were,  or,  to  be  more  accurate,  suspended, 
in  the  magma.  A  brief  consideration  of  certain  experimental  deter- 
minations shows  that  such  suspension  can  occur  in  a  normal  magma 
invading  rocks  of  average  specific  gravity  only  on  the  condition  that  the 
magma  is  highly  viscous  and  near  the  point  of  consolidation.  It  has 
been  established  that,  for  each  class  of  hoioerystalline  silicate  rocks,  the 
specific  gravity  of  the  corresponding  glass  is  considerably  lower  than 
that  of  the  natural  rock,  and  that  the  specific  gravity  of  the  same  rock 
when  completely  melted  is  still  lower  than  that  of  the  glass.  No  inves- 
tigation has  been  made  on  these  points  for  any  of  the  Ascutney  rocks, 
l)ut  it  is  fair  to  use  the  results  for  similar  rocks  from  other  parts  of 
the  world. 

The  most  important  case  for  consideration  is  evidently  the  relation 


"  Amoug  other  papers,  cf.  C.  Doelter,  Die  Schmelzbarkeit  der  Mineralien  und  ihre  Loslichkeit 
in  Magmeu.  Tscher,  Mm.  u  Petrog  Mitth..  Vol  XX,  laoi,  p  30?;  and  Ueber  emige  petrogene- 
tische  Fragen:  Centralbl.  t.  Mm.,  Geoi.  und  Pal.,  i90'«i,  p  545 


DALY.] 


SUGGESTED    HYPOTHESIS    AS    TO    THE    INTRUSION. 


95 


between  the  specific  gravity  of  the  rocks  in  the  Gneissic  series  and 
that  of  the  diorite-gabbro  magma;  therein  we  must  have  the  closest 
approximation  in  density  between  the  material  of  any  one  stock  and 
its  staple  inclusion.  The  specific  gravity  of  the  chemically  analyzed 
diorite  is  2.936.  Similar  determinations  were  made  for  the  more  basic 
gabbro  jjhases  collected  at  three  different  j)arts  of  the  same  stock ;  the 
values  here  ran  from  2.95  to  3.19.  The  average  for  the  gabbro  is  3.08. 
We  may  take  3.10  as  the  approximate  average  specific  gravity  of 
the  more  basic  parts  of  the  oldest  stock. 

The  most  thorough  and  careful  experiment  bearing  on  this  question 
is  that  made  by  Bar  us  in  the  fusion  of  diabase.^'  He  found  that  a 
sample  of  diabase  at  20°  C.  had  a  specific  gravity  of  3.0178  and  the 
glass  produced  by  the  dry  fusion  of  the  same  rock  had,  at  the  same 
temperature,  a  specific  gravity  of  2.717.  The  density  was  much  less 
in  the  molten  state.  Thus,  at  1,400°  C.  the  specific  gravity  was  only 
2.523,  corresponding  to  an  increase  of  volume  of  about  20  per  cent.  A 
critical  discussion  of  many  fusion  experiments  by  Delesse  and  Cossa 
along  the  same  line  shows  a  close  agreement  in  the  behavior  of  the 
basic  rocks  treated  in  the  older  researches,  as  compared  with  that  of 
the  diabase  of  Barus's  refined  experiment.^  One  phase  of  the  corre- 
spondence is  shown  in  the  following  table : 


Rock  type. 

1. 

Sp.gr.  of 
rock  at 
ca  20°  C. 

Sp.gr.  of 
glass  at 
ca  20°  C. 

3. 

Net  de- 
crease in 
density, 
rock  to 
glass. 

4. 

Net  in- 
crease in 
volume, 
rock  to 
glass. 

5. 
Sp.  gr.of 
rock  molten 
at  1,400°  C, 
calculated 
from  Ba- 
rus's fusion 
curve. 

Diabase  of  Bams 

3.0178 

2.  999 

2.859 

2.667 

2.710 

2.684 

2.717 
2.652 
2.657 
2.403 
2. 430 
2.438 

Percent. 

10.00 

11.57 

7.07 

9.90 

10.33 

9.16 

Per  cent. 
11.2 
13.1 
7.6 
11.1 
11.5 
10.0 

2. 523 

Average  gabbro  of  Delesse-  _ 
Average  diorite  of  Delesse .  - 

Quartz-diorite  of  Cossa 

Syenite  of  Cossa 

2.507 
2.390 
2.229 
2.266 

Average  granite  of  Delesse  . 

2.243 

Average  of  above 

9.67 
6.95 

10.7 

7,5 

Gneiss  of  Delesse    - 

2.821 

2.625 

2.358 

It  is  seen  that  these  various  independent  investigations  establish  a 
tolerably  constant  ratio  for  the  i-elative  volumes  of  a  holocrystallme, 
plutonic  rock  and  of  the  glass  produced  by  its  fusion.  Of  special 
interest  are  the  small  differences  among  the  results  of  Barns,  Delesse, 
and  Cossa  on  diabase,  gabbro,  and  quartz-diorite.  These  are  rocks 
related  to  various  facies  of  the  Basic  stock  at  Ascutney  Mountain. 

aPhilos.  Mag.,  Ser.  V,  Vol.  XXXV,  1893,  p.  173;  and  Bull.  U.  S  Geol,  Survey  No,  103,  1»93.  Cf. 
Joly,  on  fusion  of  basalt,  Trans  Roy.  Dublin  Soc,  Ser.  II,  Vol.  VI,  1897,  p.  298 

6 Delesse,  Bull.  Soc,  Geol  France,  Ser,  II,  Vol.  IV,  1847,  p.  1380;  Cossa,  ref.  by  Zirkel,  Lehrbuch 
der  Petrographie,  Vol.  1, 1893,  p  681 


96  GEOLOGY    OF    ASCUTKEY    MOUNTAIN,   VEEMONT.  [bull.  209. 

The  behavior  of  all  basic  rocks  iindei-  fusion  has,  unfortunately,  not 
been  tested  for  high  temperatures,  but,  for  reasons  well  established  by 
Barus  and  derivable  from  a  survey  of  this  particular  field  of  research, 
it  is  admissible  to  apply  the  fusion-curve  of  Barus  to  any  rock  of 
allied  composition.  On  this  supposition,  at  one  atmosphere  of  pres- 
sure, the  specific  gravity  of  the  Aseutney  gabbro  would  fall  from  3.10 
to  2.59  at  1,400°  C.  and  that  of  the  average  diorite  from  2.94  to  2.46. 
At  this  temperature  the  rock  would  remain  highlj^  fluid  even  at  the 
depth  of  5  miles  in  the  earth's  crust.  The  specific  gravity  of  the  nor- 
mal gneisses  occurring  near  the  Basic  stock  ranges  from  about  2.69  to 
about  2.76,  with  a  probable  average  of  2.73. 

To  determine  what  these  would  be  if  fragments  with  the  correspond- 
ing densities  could  be  kept  solid  and  obey  the  law  of  expansion  for 
solid  rock,  at  1,400°  C,  it  is  permissible  to  use  Reade's  expansion  coeffi- 
cient for  granite  without  incurring  serious  errror.* 

Barus  has  shown  that  pressure  simplj^  elevates  the  melting  point  in 
the  normal  type  of  fusion  without  interfering  essentially  with  the 
value  of  the  coefficient  determined  at  ordinar}^  temperatures  and 
pressures.*  The  average  gneiss  would  have,  as  a  result  of  the  appli- 
cation, a  calculated  specific  gravity  of  2.63.  It  must  be  remembered, 
too,  that  contact  metamorphism  here,  as  generally  elsewhere,  would 
raise  this  value  still  higher,  and  that  any  acidification  of  the  magma 
in  contact  with  the  gneiss  would  lower  the  density  of  the  magma. 
Now,  the  beautiful  experiments  of  Barus  in  the  fusion  of  various  car- 
bon compounds  under  varying  pressures  show  that,  in  thermal  expan- 
sibility and  in  compressibility,  they  behave  in  a  manner  extremely 
similar  to  the  few  silicates  on  which  any  studies  in  fusion  have  been 
made.  He  has  shown  that  naphthalene,  a  substance  obejang,  like 
diabase,  the  normal  law  of  fusion,  is  slightly  more  compressible  as  a 
liquid  than  as  a  solid.''  The  fusion  curves  indicate  that,  for  the  same 
increase  of  pressure,  liquid  naphthalene  gains  in  specific  gravity  about 
twice  as  fast  as  solid  naphthalene.  The  compressibility  of  a  fused  sili- 
cate rock  is  probably,  then,  approximately  twice  that  of  the  same  rock 
when  solid.  But  his  diabase  curve  demonstrates  that  the  thermal 
ex]3ansibility  of  the  liquid  rock  is  1.9  as  rapid  as  that  of  the  solid  rock. 
Thus  a  block  of  cold  solid  gabbro  immersed  in  a  deep-seated  molten 
magma  of  the  same  chemical  composition  would  be  less  condensed  by 
the  pressure  than  the  molten  rock,  but  the  effect  on  relative  densities 
would  be  partly  compensated  by  the  relative  rate  of  expansion  due 
to  any  superheating  of  the  magma.  A  block  of  gneiss  would  behave 
in  a  manner  closel}^  similar  to  that  of  a  block  of  gabbro.  It  is  believed 
that  the  pressure  of  several  thousand  atmospheres  would  not  affect 
seriously  the  contrast  in  densities  which  experiment  would  lead  us  to 
expect  if  a  fragment  of  the  Aseutney  gneiss  were  completely  immersed 
in  the  fused  gabbro  at  plutonic  pressures.     If  this  be  true,  only  one 

a  Origin  of  Mountain  Ranges,  London,  1886,  p.  110.      h  Philos.  Mag.,  Vol.  XXXV,  1893,  p.  306. 
c  Am.  Jour.  Sci.,  3d  ser..  Vol.  XLII,  1891,  p.  140. 


DALY.] 


SUGGESTED    HYPOTHESIS    AS    TO    THE    INTRUSION. 


97 


conclusion  can  be  drawn.  Since  uniform  pressure  affected  botli 
gneissic  fragment  and  magma  when  the  former  was  j)arted  from 
the  parent  country  rock,  the  difference  of  density  of  the  two  would 
prevent  the  suspension  of  the  fragment  as  a  mere  matter  of  flotation. 
Further,  the  fragments,  like  the  basic  segregations,  could  remain  in 
the  positions  in  which  they  may  now  be  seen  only  if  the  magma 
possessed  a  high  viscosity  at  the  time  when  they  were  rifted  off. 

If  we  are  forced  to  this  view  of  the  conditions  in  the  Basic  stock, 
still  more  surely  may  we  have  confidence  in  it  as  explaining  the 
presence  of  even  more  numerous  schist  fragments  in  the  syenitic  and 
granitic  stocks.  The  following  table  shows  that,  even  in  the  holo- 
crystalline  state,  each  irruptive  rock  has  a  specific  gravity  lower  than 
its  country  rocks.  It  indicates  further  that  the  inequality  increases 
in  the  same  sense  the  greater  the  degree  of  exomorphic  change  in  the 
invaded  schist. 


Eruptive  rock. 

Specific  gravity. 

Corresponding  country  rocks. 

Approximate 
specific 
gravity. 

Normal  sericitic  schist 

2.70 

Main  stock 

(2.  616 -2. 683' 
laverage,  2. 65 

Average  of  three  specimens 
from  phyllite  of  contact 
zone. 

2.84 

Average  of  Basic  stock 

3.05 

Nordraarkite-p  o  r  p  h  yr  y 
of  Little  Asciitney. 

I             2. 633 

(•Average  of  Basic  stock 

I  Average  of  gneisses 

3.05 

2.73 

Pulaskite,  Pierson  Peak. 

about  2. 63 

Average  of  Basic  stock 

3.05 

rNormal  sericitic  schist 

2.68 

Granite  stock .         

2.616 

Average  of  hornfels  from 
phyllitic  contact. 

2.84 

-Average  of  Main  syenite .  _ . 

2.65 

Delesse  found  that,  in  melting  down  granite  to  a  glass,  the  specific 
gravity  was  lowered  about  10  per  cent  on  the  average.'^'  Accepting 
his  figure,  the  biotite-granite  of  Ascutney  would  afford  a  glass  with  a 
specific  gravity  of  about  2.35.^  A  block  containing  1,000  cubic  feet  of 
the  porphyritic  phase  of  the  Main  syenite  would  tend  to  sink  in  a  magma 
of  the  latter  specific  gravity  by  virtue  of  a  downward  pull  equal  to 
the  weight  of  at  least  5.3  tons  of  rock  in  the  air,  and  evideutly,  from 
Barus's  results,  still  faster  in  the  tliinly  fluid  granite  itself.  It  is  in 
the  highest  degree  probable  that  this  difference  of  density  would  not 
be  significantly  altered  by  the  great  pressures  reigning  at  the  moment 
when  such  a  block  would  become  detached  from  the  wall  of  the  granite 
bodj^     Nothing  less,  then,  than   a  very  unyielding,  highly  viscous 


«Annales  des  Mines,  Ser.  II,  Vol.  IV,  1847,  p.  1380, 
''Cf.  granite,  specific  gravity  2.63;  obsidian,  2.3  to  2.4. 

Bull.  209—03 7 


98  GEOLOQ-Y    OF    ASOUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 

condition  of  any  one  of  the  Ascutney  magmas  can  account  for  the 
presence  of  the  foreign  blocks  in  the  immediate  vicinity  of  their 
homes  in  the  invaded  formations.  The  viscosity  probably  approached 
that  of  the  Archean  granitic  magmas,  which,  according  to  Lawson, 
were  capable,  under  enormous  djmamic  stresses,  of  shearing  and  atten- 
uating foreign  blocks  suspended  in  those  magmas  near  the  moment  of 
consolidation  of  the  latter.  Lawson  has  also  suggested  that,  although 
the  viscosity  was  so  great,  the  temperatures  may  have  been  high 
enough  to  melt  up  the  more  basic  foreign  fragments  completely.^' 
Whether  solid  or  molten  when  sheared  or  pulled  out,  such  blocks 
could  not  sink  in  the  magma,  because  of  its  thick,  pasty  condition. 

At  Ascutney  Mountain,  as  elsewhere,  the  magmas  that  formed  the 
stocks  were  capable  of  forcing  their  way  through  fissures  a  few  inches 
or  but  a  fraction  of  an  inch  in  width,  for  distances  of  hundreds  of  feet 
or  yards  from  the  respective  main  eruptive  mass.  These  are  clearly 
offshoots  from  the  stocks,  though  the  junction  with  the  latter  may 
not  be  seen  in  many  instances.  Each  magma  must  have  been  very 
fluid  when  it  filled  its  own  set  of  these  narrow  fissures.*  That  con- 
clusion accords  with  the  results  of  the  recent  careful  experiments  of 
Doelter.^'  He  has  shown  that  there  are  but  comparatively  small  differ- 
ences among  the  temperatures  at  which  a  granitic  rock  or  an  artificial 
mixture  of  silicates  is  softened  by  heat,  becomes  thinly  molten,  or 
solidifies  from  that  molten  condition.  Thus  he  found  that  a  foyaitic 
mixture  (of  orthoclase,  el?eolite,  and  tegirine)  became  soft  at  1,070°  C, 
thinly  fluid  at  1,110-1,115°,  and  then  solidified  at  980-1,000°  C.  The 
corresponding  figures  for  a  basaltic  mixture  (of  labradorite,  augite, 
olivine,  and  magnetite)  are  1,120-1,125°,  1,140-1,150°,  and  980-1,000°  C. 
Predazzo  granite  and  Remagen  basalt  became  softened  at  respective 
temperatures  of  1,150°  and  992°  C. ;  completely  molten  at,  respectively, 
1,240°  and  1,060°  C.  But  a  slight  restoration  of  heat,  therefore,  would 
be  necessary  to  reconvert  a  cooled  and  toughly  viscous  endomorphic 
zone,  yet  hot  enough  to  quarry  blocks  from  the  invaded  formation, 
into  a  highly  mobile  state.  It  can  not  be  denied  that  there  must 
occur  a  loss  of  at  least  that  small  amount  of  heat  in  the  closing  stage 
of  stock  intrusion.  The  magnitude  of  plutonic  pressures  puts  no 
difficulty  in  the  way  of  accepting  this  conclusion  as  to  high  fluidity. 
Oetling  has  proved,  on  the  contrary,  that  the  temperature  point  of 
consolidation  of  melted  rocks  and  silicate  mixtures  is  lowered  by 
pressure.  He,  in  fact,  shares  the  view  of  Amagat,  that,  if  the  pressure 
be  sufficiently  high,  solidification  can  not  occur  at  all.^^    Moreover,  it 

«Geol.  Rainy  Lake  Region,  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Canada,  1887,  Part  F,  pp. 
131-2-3-8,  etc. 

''There  is  no  contradiction  between  this  statement  and  the  previous  one  of  high  viscosity  in 
the  main  magma  which  isolated  and  suspended  foreign  blocks.  As  implied  in  a  following  para- 
graph, it  would  simply  mean  that  the  apophysal  tongues  were  injected  before  the  magma  had 
come  in  contact  with  the  present  walls  of  its  miain  chamber. 

c'Tscher.  Min.  u.  Petrog.  Mitth.,  Vol.  XX,  1901,  pp.  333  and  307 

rflbid..  Vol.  XVII,  1897,  p.  370. 


DALY.]  SUGGESTED    HYPOTHESIS    AS    TO    THE    INTRUSION.  99 

is  coming  to  be  generally  accepted  that  pressure  induces  mobility  in 
plntonic  magmas  by  retaining  water  and  mineralizers." 

In  summary,  tlien :  Field  observation  and  experiment  agree  in  at- 
tributing a  thinly  fluid  condition,  except  at  the  moment  of  final  crys- 
tallization, to  such  magnias  as  those  from  which  the  Ascutney  stocks 
were  derived.^ 

High  fluidity  must  have  two  important  results:  First,  it  would 
facilitate  the  formation  of  ax)ophysal  tongues,  often  intersecting; 
secondly,  it  would  entail  a  downward  strain  on  any  disjointed  blocks 
in  the  roof  contact  of  the  stock  body.  Following  joints,  planes  of 
stratification,  schistosity,  or  slipping,  the  apophyses  must  seriously 
impair  the  strength  of  the  roof  and  walls.  The  same  planes  of  weak- 
ness, even  without  the  aid  of  the  irruptive  wedge,  already  form  a 
menace  to  the  integrity  of  the  walls  and  roof,  especially  the  latter; 
this  on  account  of  the  gravit}^  component  already  demonstrated  as  a 
result  of  a  difference  of  density  between  the  solid  rock  and  the 
magma  beneath.  Moreover,  a  shattering  of  the  country  rock  may  be 
expected  by  reason  of  the  differential  temperature  strains  induced  by 
the  magma. 

When,  from  these  causes,  a  block  becomes  dislodged  and  completely 
immersed  in  magma,  it  must  sink,  and  sink  rapidly.'^  The  space 
formerly  occupied  by  the  block  is  now  filled  mth  magma.  In  the 
same  manner  an  indefinite  number  of  blocks  may  be  removed  by  this 
natural  stoping.  New  surfaces  will  continually  be  presented  to  the 
invading  magma,  and  so  long  as  the  stated  conditions  persist  there  will 
be  greater  and  greater  destruction  of  the  country  rock.  It  is  simply  a 
question  of  time  whether  the  advance  of  the  magma  shall  be  so  great 
as  to  fashion  the  chamber  of  an  Ascutney  stock  or  of  a  great  batholith. 

A  brief  statement  of  this  central  idea  of  the  stoping  hypothesis 
has  been  given  by  Lawson  in  a  review  of  certain  of  Brogger's  writ- 
ings. So  far  as  known  to  the  present  writer  this  noteworthy  para- 
graph contains  the  only  clear  enunciation  of  the  doctrine  to  be  found 
in  geological  literature,  and  is  worth j^  of  quotation  in  full: 

The  essential  features  of  tlie  assimilation  hypothesis  were  formulated  by  the 
reviewers  some  years  agO;  before  the  publication  of  Michel  Levy's  views,  and 
urged  as  a  satisfactory  explanation  of  the  remarkable  relations  which  obtain 
between  the  Laurentian  granites  and  gneisses  and  the  upper  Archean  or  Ontarian 
metamorphic  rocks.  These  intrusive  granites  and  gneisses  occupy  vast  tracts  of 
the  Canadian  Archean  plateau,  and  there  seems  to  be  no  escape  from  the  view 
that  they  bear  a  bathohtic  relation  to  the  crust  which  they  invaded  from  below. 
Portions  of  the  crust  were  absorbed,  but  there  are  two  possibilities  as  to  the 
method  of  absorption,  viz:  1,  by  fusion;  2,  by  sinking  into  the  magma.  The 
numerous  blocks  of  rock  scattered  through  the  granites  lend  much  probability 

aDoelter,  Tscher.  Min.  u.  Petrog.  Mitth.,  Vol.  XXI,  1902,  p.  218. 

&See  the  general  statement  by  Brogger,Die  Evuptivgesteine  des  Kristianiagebietes,  Vol.  Ill, 
p.  338. 

c  Johnston-Lav  is  has  seen  a  piece  of  compact  lava  sink  quickly  in  a  flowing  lava  stream  from 
Vesuvius.    Proc.  Quart.  Jour.  Geol.  Soc,  Vol.  XXXVIII,  1882,  p.  240. 


'100  GEOLOaY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull.  209. 

to  the  latter  having  played  a  part  in  the  process.     Such  batholiteswere  doubtless 
accompanied  by  laccolitic  satellites. « 

The  hypothesis  of  natural  overhead  "  stoping "  accords  with  the 
facts  known  with  regard  to  other  kinds  of  igneous  intrusion.  Even 
in  the  case  of  those  great  granitic  massifs  organically  associated  with 
master  lines  or  zones  of  dislocation  (e.  g.,  the  tonalite  and  the  "Judi- 
carienlinie"  of  the  Tyrol),  the  magma  chamber  may  have  been  largely 
opened  by  overhead  stoping.  The  same  process  may  similarly 
greatly  enlarge  the  deep-seated  cross  section  of  a  volcanic  neck. 
Yet  no  one  can  deny  its  practical  insignificance  in  the  intrusion  of 
sheets  or  dikes,  nor,  for  obvious  reasons,  does  that  fact  injure  the 
strength  of  the  proposed  hypothesis  when  dealing  with  vastly  larger 
igneous  bodies.  The  latter  must  be  much  longer  molten  by  reason  of 
their  size,  and  have  more  direct  communication,  through  convection 
and  other  currents,  with  the  earth's  interior.  The  same  remark  apr 
plies  in  general  to  laccoliths,  although  it  is  possible  that,  in  limited 
degree,  laccolithic  magmas  maj^  carry  on  independent  stoping,  and 
therewith  assimilation,  in  their  hot  interiors. 

The  hypothesis,  it  will  be  observed,  is  allied  in  one  respect  to 
the  assimilation  theory  of  Kjerulf,  Michel  Levy,  Lacroix,  and  others. 
According  to  each  of  the  two  views,  the  plutonic  chamber  occupied 
by  stock  or  batholith  has  been  formed  by  the  activity  of  the  magma 
itself  along  the  internal  contact.  But,  in  the  older  theory,  the  assimi- 
lation at  the  contact  is  essentially  caustic  and  chemical;  in  the  newer 
view  the  assimilation  there  is  essentially  mechanical.  The  former 
attempts  to  explain  in  one  step  the  opening  of  the  space  now  filled 
with  eruptive  material  and  the  disappearance  of  the  corresponding 
mass  of  country  rock ;  the  latter  has  still  to  give  account  of  the  multi- 
tude of  larger  or  smaller  blocks  sunken  in  the  magma.  What  becomes 
of  them?    How  far  will  they  sink?     What  is  their  fate  when  they 

come  to  rest? 

ABYSSAL  ASSIMIIjATIOIS^. 

It  is  at  once  evident  that  such  questions  are  most  difficult  to  answer 
in  detail ;  perhaps  the  second  is  always  destined  to  remain  unanswered. 
It  is  evident,  too,  that  we  are  now  many  removes  nearer  the  realm  of 
speculation  than  in  any  previous  explanatory  step.  Yet  it  can  not 
be  considered  a  fatal  objection  to  any  theory  of  intrusion  that  it  must 
refer  ultimately  to  the  unexplored  interior  of  the  earth.  The  attempts 
to  solve  the  plutonic  problem  with  attention  rigidly  kept  on  the  acces- 
sible part  of  the  earth's  crust  must  have  but  partial  success.  If 
experiment,  analogy,  and  the  considerations  of  cosmical  physics  can 
aid  in  exx^lanation,  they  should  be  employed.  The  field  geologist  has 
only  the  earth's  outer  skin  to  study;  yet  granite,  with  all  its  relatives, 
is  a  product  of  physiological  processes  occurring,  as  it  were,  in  the 
vital  organs  of  the  earth. 

The  cardinal  fact  of  fluidity  in  plutonic  magmas  needs  to  be  viewed 

"Science,  new  series,  Vol.  III.,  1896,  p.  637. 


DALY.]  ABYSSAL    AbSIMTLATION.  101 

iu  relation  to  the  equally  certain  fact  of  the  earth's  rigidity  and  to 
the  necessity  of  finding  some  mechanical  explanation  for  the  support 
of  the  roof  over  the  igneous  body  during  intrusion.  A  complete  dis- 
cussion of  the  former  topic  would  carry  us  farther  afield  than  the 
scope  of  the  present  report  warrants.  Suffice  it  here  to  note  that  the 
same  problem  confronts  every  modern  theory  of  intrusion. 

In  the  case  of  the  Ascutney  stocks  it  is  believed  that  the  strength 
of  the  roof  over  each  irruptive  mass  was  doubtless  sufficient  to  pre- 
vent its  foundering  en  masse  in  the  less  dense  magma.  Other  and 
larger  stocks  and  batholiths  must  be  studied  in  this  regard  each  by 
itself.  As  the  underpinning  of  the  schist  cover  of  the  Ascutney 
igneous  area  as  a  whole  was  demonstrably  aided  by  a  progressive  con- 
solidation of  the  partial  magmas,  so  it  is  conceivable  that  there  may 
be  a  lateral  progression  of  solidification  in  the  homogeneous  magma 
of  a  much  larger  body  with  a  corresponding  strengthening  of  its  roof. 
In  all  such  intrusions  there  will  also  be  the  continued  presence  of 
country-rock  buttresses  still  remaining  unassimilated. 

Whether  a  stoped-out  block  sinks  in  the  magma  but  thousands  of 
feet  or  miles  from  its  former  position  in  roof  or  wall,  that  block  must 
undergo  an  increase  of  pressure,  and,  with  the  greatest  probability, 
an  increase  of  temperature.* 

The  added  pressure  would  have,  according  to  the  experiments  and 
field  studies  of  Barns,  Doelter,  Daubree,  Fouque,  Michel  Levy,  and 
others,  the  secondary  effect  of  increasing  in  the  magma  the  capacity 
of  retaining  water  and  other  solvents,  even  at  very  high  temperatures.^ 
So  important  are  other  experiments  in  this  connection  that  a  brief 
resume  of  certain  results  accruing  from  them  must  be  given. 

The  solubility  of  rock-forming  minerals  in  silicate  magmas  has  been 
shown  by  fusion  experiments  to  depend  on  (a)  the  temperature  of 
the  magma;  (5)  the  chemical  com]3osition  and  fluidity  of  the  magma; 
(c)  the  fusibility  of  the  minerals,  and  {d)  on  jjressure.  Doelter  has 
been  able  to  prove  that,  under  one  atmosphere  of  pressure,  all  the  com- 
mon types  of  rock-forming  minerals  are  completely  soluble  in  certain 
representative  magmas  at  temperatures  only  slightly  above  those  of 
the  respective  consolidation  points  of  the  latter.  These  magmas  were 
made  from  granite,  obsidian,  common  basalt,  limburgite,  phonolite, 
foyaite,  leucite-basalt,  leucitite,  hornblende-andesite,  and  nepheline- 
basalt — a  magmatic  range  so  wide  as  to  demonstrate  the  practical 
certainty  that  all  silicate^  magmas  have  similar  solvent  properties. 
He  further  shows  that  the  melting  point  of  a  silicate  rock  occurs  at 
about  the  average  temperature  of  fusibility  of  its  constituent  min- 
erals. Long  before,  Bischof  easily  dissolved  clay- slate  in  fluid  lava, 
using   a  bellows   furnace  for  fusion.^    These  important  deductions 

"Perhaps  the  block  would  sink  to  the  zone  of  pressure-solid  magma. 

6Among  the  more  recent  papers,  cf.  C.  Barus,  Am.  Jour.  Sci.,  Vol.  XXXVIII,  1889,  p.  408,  and 
Vol.  XLI,  1891,  p.  110;  C.  Doelter,  Centralbl.  f.  Min.,  etc.,  1903,  p.  550,  and  Tscher.  Min.  u.  Petrog. 
Miith.,  Vol.  XXI,  1902,  p.  218. 

'Chem.  u.  Phys.  Geol.,  Supplement,  1871,  p.  98. 


102  GEOLOGY    OF   ASCUTNEY   MOUNTAIN,   VERMONT.         [bull.  209. 

from  laboratory  investigations  correspond  to  the  facts  of  outdoor 
nature.  Well-known  practical  examples  may  be  found  in  the  fused 
and  greatly  corroded  granite  inclusions  in  the  basalts  of  the  Auvergne, 
and  again  in  the  complete  disapjDearance  by  fusion  of  the  "floating 
islands"  in  the  caldera  of  Kilauea.^'  The  high  fluidity  of  the  normal 
plutonic  magma  would  likewise  facilitate  the  complete  solution  of 
foreign  fragments,  as  experimentally  proved  by  Doelter. 

It  is  true  that  the  direct  influence  of  pressure  is  directed  toward 
elevating  the  melting  points  of  silicate  mixtures,  though  probablj^ 
not  in  a  degree  proportional  to  the  amount  of  the  pressure.*  Yet 
that  effect  on  the  solvent  power  of  the  magma  may  be  much  more 
than  counterbalanced  by  the  indirect  effect  of  pressure  in  retaining 
water  and  other  solvents.  Once  molten,  pressure  tends  to  keep  sili- 
cate magmas  molten,  since  it  lowers  the  temperature  point  of  consoli- 
dation.^ In  determining  the  solvent  power  of  a  plutonic  magma, 
temperature  furnishes  here,  as  in  fixing  the  melting  point,  the  "  coarse 
adjustment,"  as  i)ressure  furnishes  of  itself  the  "fine  adjustment." 

In  conclusion,  then,  it  seems  legitimate  to  regard  the  conditions  of 
the  abyssal  portions  of  plutonic  magmas  as  conspiring  toward  the 
perfect  digestion  of  a  submerged  foreign  rock  fragment  during  all  the 
time  of  intrusion  except  during  the  short  period  preceding  final  con- 
solidation. Even  so  uncompromising  an  opponent  of  the  theory  of 
contact  digestion  by  stock  magmas  as  Brogger  admits  that  such 
assimilation  can  be,  in  the  greater  depths,  exceedingly  important, 
' '  ausserordentlich  bedeutend. "  '^ 

Since  it  is  ]3robable  that  magmas  are  more  or  less  completely  satu- 
rated solutions,^  there  would  doubtless  be  a  volumetric  increase  on 
the  fusion  of  each  block  at  whatever  depth  it  attained,  an  increase  com- 
parable to  that  demonstrated  in  fusion  experiments  at  1  atmosphere  of 
pressure.  The  question  at  once  arises  as  to  what  compensation  can 
be  made  for  the  increased  bulk  of  rock  matter  below  the  earth's  sur- 
face incident  to  abyssal  assimilation  on  a  large  scale.  Two  possibil- 
ities suggest  themselves  in  the  face  of  the  hj^drostatic  problem  involved. 
Either  volcanic  outflow  elsewhere  or  secular  uplieaval  in  the  region 
would  satisfy  the  conditions.  The  latter  would  seem  to  be  more 
likely  of  fulflllment  in  regard  to  stocks  and  batholithic  intrusions  gen- 
erally. It  is  to  be  noted  that  magmatic  stoping  would  tend  to  weaken 
the  earth's  crust  immediately  above  the  intruding  body,  and  there 
secular  elevation  of  the  surface  would  be  particularly  looked  for. 
There  may,  in  this  way,  be  found  one  cause  of  the  huge  buckles  filled 
with  the  "central  granites  "  of  Alpine  mountain  chains.     This  implies 


«.J.  D.  Dana,  Characteristics  of  Volcanoes,  New  York,  1891,  p.  176. 
b  Doelter,  Tscher.  Min.  ti.  Petrog.  Mitth.,  Vol.  XXI,  1902,  p.  221. 
cOetling,  op.  cit.,  p.  370. 

rtDie  Eruptivgesteine  des  Kristianiagebietes,  Vol.  Ill,  1898,  p.  350. 

eLagorio,  Tscher.  Min.  u.  Petrog.  Mitth.,  Vol.  VIII,  1887,  p.  504.    Cf.  Delesse,  Bull.  Soc.  Geol. 
France,  ser.  ii,  Vol.  IV,  1847,  p.  1393. 


DALY.]  .  ABYSSAL    ASSIMILATION.  103 

that  the  doming  of  the  great  intrusive  masses  of  the  Christiania 
region,  attributed  by  Brogger  to  laccolithic  injection,  may,  in  reality, 
be  due  to  this  crustal  weakening  and  buckling  by  magmas  working 
up  from  the  "ewige  Teufe,"  but  at  present  it  must  remain  only 
the  suggestion  of  a  possibility,  as  the  writer  has  no  personal  knowledge 
of  the  region. 

It  is,  moreover,  worthy  of  inquiry  whether  this  sort  of  live  energy 
of  intruding  granitic  magma  may  be  responsible  for  many  of  the 
well-known  cases  where  the  secondary  structure  planes  in  the  in- 
vaded formations  v/rap  around  their  respective  intrusive  bodies. 
Examples  are  seen  in  the  highly  developed  peripheral  cleavage  and 
schistosity  parallel  to  the  outlines  of  such  magmas  in  the  Rainj^  Lake 
region '^  and  in  the  Black  Hills.*  Such  structures  could  certainly  be 
produced  by  the  force  of  magmatic  expansion,  provided  that  force  be 
sufficient  in  amount,  for  it  must  be  exerted  alwaj^s  normal  to  the 
chamber  walls. 

If  the  foregoing  reasoning  is  correct,  the  ]3reparation  of  the  chambers 
within  which  the  stock  bodies  of  Ascutney  Mountain  now  rest  was 
carried  out  by  mechanical,  piecemeal  disruption  of  each  invaded  ter- 
rane  by  the  attack  of  the  magma  on  the  main  contacts.  This  physical 
action  was  accompanied  by  chemical  assimilation  at  greater  depths. 
Consequently,  at  those  depths  the  magma  must  become  more  and  more 
mixed  as  the  result  of  assimilation.  Each  successive  eruption  from 
the  magma  basin  beneath  may  be  expected  to  show  indications  of  the 
gradual  alteration  of  the  magma  by  the  incorporation  of  foreign  sub- 
stance. This  important  corollary  has  to  do  with  the  great  question  of 
the  origin  of  the  igneous  rocks,  a  subject  which,  in  spite  of  all  its  com- 
plex difficulties,  must  here  be  dwelt  upon  so  far  as  to  show  agreement 
or  disagreement  with  the  hypothesis  just  outlined.  But  a  less  impor- 
tant, although  significant,  test  of  the  hypothesis  may  first  be  noted. 

The  hypothesis  of  rifting  not  only  gives  adequate  reason  for  the  very 
general  sharpness  of  contact  between  an  irruptive  and  its  country 
rock,  but  also  goes  far  to  explain  the  observed  lack  of  enrichment  of 
the  endomorphic  zone  with  the  material  of  the  country  rock.  The 
blocks  would  be  likely  to  suffer  most  from  solution  in  the  magma  after 
they  had  begun  their  rapid  downward  journey.  Thej'^  would  yield  up 
their  substance  along  the  whole  path.  There  would  thus  be  a  tendencj^ 
toward  an  equal  distribution  of  the  absorbed  material  throughout  the 
magma.  In  any  case,  there  would  be  far  less  impregnation  of  the 
endomorphic  zone  with  the  substance  of  the  invaded  formation  than 
that  demanded  by  the  supposition  of  the  slow  digestion  of  the  latter 
in  place.  In  so  fluid  a  magma  convection  currents  would  tend  still 
further  to  destroy  any  contrast  of  composition  between  the  endo- 
morphic zone  and  the  body  of  the  intrusive. 


"Lawson,  Ann.  Rept.  Geol.  and  Nat.  Hist.  Survey  Canada,  188",  Part  F,  map. 
6  Van  Hise,  Sixteenth  Ann.  Rept.  U.  S.  Geol.  Survey,  Part  I,  1896,  pp.  637  and  81.5. 


104  GEOLOGY    OF    ASCUTNEY  MOUNTAIN,   VEEMONT.         [bull.209. 

EVIDEJfl^CES  OF  DIFFERElVTIATIOlSr. 

In  turning  to  the  main  problem  still  awaiting  us,  the  relation  of  the 
hypothesis  of  rifting,  overhead  stoping,  and  abyssal  assimilation  to 
the  sequence  of  the  eruptive  rocks  at  Ascutney  Mountain,  it  must  be 
be  stated  in  advance  that  differentiation  in  the  usual  sense  of  that 
term  has,  it  is  believed,  been  operative  in  the  production  of  these 
rocks.  This  illuminating  principle  seems  to  win  added  credibility 
every  year,  as  the  petrological  facts  concerning  consanguinity,  com- 
plementary dikes,  etc.,  become  more  numerous  and  more  clearly 
ascertained.  Without  entering  further  into  the  general  question,  the 
course  of  our  argument  demands  that  some  of  the  concrete  evidences 
for  the  value  of  the  principle  be  noted  as  the  result  of  a  study  of 
Ascutney  Mountain. 

Direct  witness  to  the  fact  of  differentiation  is  found  in  the  abundant 
and  remarkable  basic  segregations  from  most  of  the  stocks  and  dikes. 
Moreover,  the  ' '  blood  relationship  "  in  mineralogical  and  chemical 
composition  of  the  main  rock  bodies  of  Little  Ascutney,  Pierson  Peak, 
and  Ascutney  Mountain  proper,  occurring  as  they  do  in  so  strikingly 
different  geological  associations,  and  the  close  agreement  in  composi- 
tion with  the  distant  syenitic  rocks  of  Essex  County,  Mass.,  Killing- 
ton  Peak,  the  Adirondacks,  Rigaud  Mountain,  Quebec,  and  the  eastern 
townships  of  Quebec,  seem  to  indicate  that  strict  chemical  and  phys- 
ical laws,  and  not  fortuitous  similaritj^  in  the  products  of  assimilation, 
govern  the  particular  groupings  of  metals  and  oxides  found  in  the 
respective  intrusives.  The  occurrence  of  nordmarkites  in  all  of  these 
regions  must  be  regarded  as  the  result  of  the  independent  assertion  in 
each  region  of  one  and  the  same  set  of  laws  of  attraction  and  concen- 
tration in  an  originally  more  complex  rock  magma  rather  than  the 
result  of  multiplied  consolidations  of  one  great  nordmarkitic  magma 
underlying  all  this  part  of  North  America. 

Further,  the  conclusion  that  mere  assimilation  of  the  invaded 
sedimentary  terranes  by  a  magma  can  not  be  used  to  exj)lain  the 
intrusives  of  this  part  of  the  world  is  rendered  all  the  more  probable 
by  a  detailed  comparison  of  the  Ascutney  eruptives  with  those  of 
Mount  Shefford,  as  described  by  Dresser.*  The  Canadian  intrusives 
named  in  the  order  of  injection  are  essexite,  nordmarkite,  pulaskite, 
camptonite,  and  bostonite.  The  first  of  these  is  considerably  more 
alkaline  than  the  Ascutney  diorite  analyzed,  but  is  probably  close 
chemicallj^  to  phase  e  of  the  Basic  stock.  Macroscopicall}^,  the  Ascut- 
ney diorite  and  the  Shefford  essexite  are  remarkably  alike  in  general 
habit,  and  the  writer  has  seen  a  coarser  phase  of  the  latter  which  has 
the  poikilitic  bisilicates  and  other  detailed  features  of  the  Ascutney 
gabbros.  As  striking  similarity  characterizes  the  green  nordmarkites 
of  the  two  mountains.  These  facts  seem  to  prove  conclusively  that 
definite  chemical  and  physical  laws  have  governed  the  formation  of 

a  Am.  Geol.,  1901,  Vol.  XXVIII,  p.  203. 


DALY.]  EVIDENCES    OF    DIFFERENTIATION".  105 

each  special  magma  which  crystallized  after  irruption  into  the  rock 
bodies  now  exposed  to  view.  There  has  been  some  post-eruptive 
differentiation  in  the  Shefford  intrusions,  as  they  possess  basified 
endomorphic  zones.  Dresser  holds  that  the  essexite,  nordmarkite, 
and  pulaskite  form  the  filling  of  a  laccolithic  space  in  the  Lower 
Silurian  sediments  of  Shefford  Mountain.  Accepting  his  view  of  the 
mode  of  intrusion,  preemptive  differentiation  from  a  magma  origi- 
nally composed  of  a  mixture  of  these  special  magmas,  might  be 
credited  with  a  full  explanation  of  the  Shefford  rocks,  though  even 
then  the  possibility  is  quite  oiDen  that  the  original  complex  magma 
had  been  formed  by  the  considerable  digestion  and  assimilation  by  a 
still  earlier  magma,  of  the  Trenton  slates  and  other  sediments  through 
which  the  eruptions  took  place.  On  the  other  hand,  the  fact  of  some 
kind  of  assimilation  preparatory  to  differentiation  at  Ascutney  can 
hardly  admit  of  doubt. 

The  differentiation  of  the  alkaline  rocks  in  the  area,  on  the  hypothesis 
outlined  for  Ascutney,  would  be  local  and  confined  to  a  magma  which 
had  been  more  or  less  strongly  affected  by  the  "mise  en  place"  of 
the  Basic  stock.  If  we  have  anywhere  an  igneous  formation  approxi- 
mately representing  the  main  magma  whicli  underlay  the  region 
before  the  intrusion  began,  it  must  be  found  in  that  stock.  All  sub- 
sequent intrusions  might,  on  account  of  the  intermixture  of  assimi- 
lated schists,  be  exi^ected  to  show  a  divergence  from  the  original 
magma  that  would  be  the  stronger  the  later  the  corresponding  intru- 
sive appeared  in  a  series  of  eruptions.  In  other  words,  the  windsor- 
ite  dikes,  the  nordmarkites,  pulaskite,  monzonite,  paisanite,  granites, 
and  aplites  are,  by  the  hypothesis,  regarded  as  the  product  of  the 
deep-seated  assimilation  of  the  schists  followed  or  accompanied  by  the 
differentiation  of  these  related  magmatic  types  from  the  mixture  due 
to  subcrustal  digestion.  The  high  silica,  ijotash,  and  alumina  of  the 
micaceous  and  quartzose  phyllites  and  gneisses  would  explain  the 
increasing  acidity,  the  alkalinity,  and  feldspathic  character  of  these 
differentiated  products,  though  other  features  must  be  credited  to 
differentiation  alone. 

Just  how  differentiation  takes  place  is  still  to  be  reckoned  among 
the  mysteries  of  geology.  There  is  no  doubt  that  several  determina- 
tive factors  must  be  taken  into  account.  Without  in  any  way  wish- 
ing to  question  the  validity  of  the  other  causes,  the  writer  will  here 
briefly  instance  one  of  them  as  seeming  to  be  of  more  general  appli- 
cation. Rosenbusch  has  published  the  view  that  the  separation  of 
differentiated  products  may  be  due  in  part  to  the  gravitative  effect, 
whereby  the  more  acid  and  lighter  constituents  of  a  complex  magma 
become  segregated  and  float  upon  the  more  basic  and  heavier  residue.'* 
It  is  supported  by  the  valuable  observations  of  Morozewicz  in 
synthetic  experiments  and  in  the  study  of  glass  furnaces.^    Doelter 

«Mikroskopische  Physiographie  d.  Min.  u.  Gest.,  Vol.  II,  1896,  p.  552. 
bTscher.  Min.  n.  Petrog.  Mitlh.,  Vol.  XVIII,  1898,  pp.  170  and  333. 


106  GEOLOGY    OF    ASOUTNEY   MOUKTAIN,   VERMONT.         [buil.  209. 

has  pointed  out  that  such  results  adhere  to  excejjtional  cases,  both 
in  his  own  experiments  and  in  those  of  the  Russian  investigator; 
yet  their  significance  is  still  great,  since  they  agree  with  Gouy  and 
Chaperon's  theoretically  deduced  principle  of  gravitative  stratification 
in  saline  solutions,^'  as  well  as  with  some  positive  field  observations. 
For  example,  Sir  A.  Geikie  describes  the  separation  of  a  lower  layer 
of  picrite  and  an  overlying  layer  of  olivine-basalt  in  the  same  lava 
flow,  and  finds  it  probable  that  similar  differentiation  has  taken  place 
in  basic  sills. ^  It  is  at  least  worth  while  to  apply  the  gravitative 
theory  to  the  Ascutne}^  magmas,  so  far  as  to  state  briefiy  the  course 
of  events  entailed. 

By  the  separation  of  the  differentiated  products,  the  uppermost 
layer  would,  by  the  antecedent  addition  of  the  abundant  silica  from 
the  digested  schists,  become  more  and  more  acid  as  the  assimilation 
progressed.  The  aplites  and  granite  would  appear  as  the  latest 
products  (excepting  the  complementary  dikes)  of  a  differentiation 
deiDcndent  on  the  assimilation  for  its  final  expression. 

Opposed  to  the  hypothesis  is  the  more  usual  view  of  simple  differ- 
entiation as  explanatory  of  the  eruptive  sequence.  The  latter  has 
been  well  expressed  by  Brogger  for  the  similar  sequence  in  the  Chris- 
tiania  region.  He  points  out  the  general  harmony  existing  between 
the  theoretical  order  of  differentiation,  the  order  of  eruption  in  the 
province,  and  the  order  of  crystallization  in  the  various  rocks. '^  On 
the  same  principle  the  oldest  Ascutney  stock  would  be  regarded  as 
of  the  nature  of  a  gigantic  basic  segregation  which  had  absorbed  into 
itself  the  basic  orthosilicates  and  metasilicates  of  lime,  magnesia,  and 
iron  before  the  crystallization,  from  the  same  original  magma,  of  the 
syenites  and  granite  where  the  dark-colored  constituents  are  so  poorly 
represented.'^  The  possibility  of  mere  differentiation  (without  assimi- 
lation) producing  the  Christiania  rock  bodies  is  due,  according  to 
Brogger,  to  the  peculiar  laccolithic  nature  of  the  intrusions  in  that 
province.  The  preparation  of  free  space  for  the  play  of  chemical 
reactions  leading  to  differentiation  is  quite  in  contrast  with  that 
hypothecated  for  the  Ascutney  area,  though  many  of  the  rock  types 
of  the  two  regions  are  extremely  similar.  Brogger  has  pronounced 
against  the  assimilation  theorj^  of  Kjerulf  and  Michel  Levy,  largely 
for  the  reason  that  h  fails  to  meet  the  controlling  test,  the  proof  of 
chemical  sympathy  between  the  formation  invaded  and  the  igneous 
body  supposed  to  have  performed  the  digestion.  Thus  the  granite  of 
the  Christiania  region  contains  scarcely  0.5  per  cent  of  CaO,  although 
the  Cambrian  and  Silurian  beds  through  which  the  intrusions  occurred 
contain  as  large  an  average  as  24.5  per  cent  of  the  same  oxide. '^  The 
objection    does    not,   however,    apply  to   the  modified   assimilation 

aAnn.  de  Chimie  et  de  physique,  6th  ser.,  Vol.  XII,  1887,  p.  384. 

b  Ancient  Volcanoes  of  Great  Britain,  London,  1897.  Vol.  I,  pp.  419  and  443,  and  Vol.  II,  p.  310. 

cDie  Eriiptivgesteine  des  Kristianiagebietes,  Vol.  II,  1895,  p.  175. 

dCl.  Zeit.  fur  Kryst.,  Vol.  XVI,  1890,  p.  86. 

eDie  Eruptivgesteine  des  Kristianiagebietes,  Vol.  II,  1895,  p.  1^9. 


PALY.]  THE    PETEOGENIC    CYCLE.  107 

hypothesis  as  outlined  in  this  chapter.  Brogger's  own  cross  sections 
would  imply  that  the  Cambrian  and  Silurian  limestones  were  depos- 
ited on  Archean  crystalline  schists.  The  vertical  thickness  of  this 
formation  is  probably  several  times  as  great  as  the  thickness  of  all 
the  Lower  Paleozoic  limestones  combined.  Differentiation  working 
on  the  magma  produced  by  the  mixture  of  the  digested  material  of 
both  limestones  and  schists  might  very  well  give  a  granite  with  a  low 
content  of  lime.  Be  the  method  of  intrusion  what  it  will,  the  simi- 
larity of  the  Norwegian  and  Vermont  rocks  seems  to  ijoint  unmistak- 
ably to  the  truth  of  the  main  principle  of  differentiation — the  tendencj^ 
toward  definite  chemical  and  mineralogical  segregation  in  a  silicate 
magma,  irrespective  of  how  that  magma  was  prepared. 

As  the  specific  gravity  of  the  acid  magmas  must  in  every  case  be 
lower  than  that  of  the  original  basic  magma,  the  latter  would  tend  to 
rid  itself  continually  of  the  foreign  substance  being  dissolved  from 
the  sunken  blocks.  We  have  seen  that  the  latter  would  sink  deepl}^ 
Whether  this  gravitative  cleansing  be  perfect  or  not  at  a  moderate 
depth  of,  perhaps,  a  mile  or  two  below  the  original  magma  surface, 
the  magma  might  there  still  be  quite  basic.  If  we  now  imagine  a  i3ro- 
longed  period  during  which  the  overlying  acid-alkaline  intrusives  were 
completely  crystallized  and,  afterwards,  a  limited  fracturing  of  the 
Avliole  compound  terrane,  we  can  secure  some  explanation  of  the  final 
series  of  basic  dikes.  They  would  represent  the  product  of  renewed 
eruptive  activity  from  the  deep-lying,  still  molten  magma  pressed 
upward  along  the  easy  paths  of  the  fractures.  The  common  occur- 
rence of  the  diabase  dikes  and  lavas  through  the  whole  length  of  the 
Connecticut  Valley  and  in  many  parts  of  the  Appalachian  system 
suggests  correlation  with  this  hypothetical  explanation.  Possibli' 
the  camptonites  are  nothing  more  than  dikes  of  diabase  which 
have  absorbed  a  small  amount  of  ferrous  iron  and  alkalies  from  the 
syenites  through  which  they  have  found  their  way.  Nevertheless,  in 
spite  of  the  diificulty  of  determining  the  place  and  exact  manner  of 
the  differentiation  of  complementary  dikes  in  general,  the  possibility 
that  these  youngest  dikes  correspond  to  the  basic  poles  of  secondary 
differentiation  can  not  be  excluded.  Nor  is  it  necessary  to  the 
hypothesis  of  abyssal  assimilation  that  either  alternative  be  estab- 
lished, for  the  hypothesis  must  be  linked  with  the  belief  in  secondary 
differentiation. 

THE   PETROGENIC   CYCLE. 

Finally,  it  should  be  observed  that  the  whole  series  of  events  lead- 
ing from  the  beginning  of  the  invasion  of  the  oldest  stock  to  the 
irruption  of  the  youngest  stock  and  dikes  might,  after  the  solidifica- 
tion of  the  last  of  these,  be  followed  by  a  resumption  of  plutonic 
activity.  There  might  thus  be  relocated  the  sequence  of  changes 
memorialized  in  the  existing  rock  bodies — basic  to  acid  through  inter- 
mediate types.     Or  any  part  of  the  cycle  might  be  repeated,  whereby 


108  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT.         [bull.209. 

relatively  basic  irruptions  into  the  schists  would  be  followed  by  more 
acid  ones.  Or,  thirdly,  the  cycle  represented  in  the  unsqueezed  igneous 
rocks  of  the  present  mountain  might  have  been  preceded  by  an 
older  cycle,  the  records  of  which  are  still  buried  deep  within  the 
schistose  formations  in  the  neighborhood.  Such  an  earlier  cycle 
would  account  for  the  amphibolites  and  aplitic  sheets  which  antedate 
the  last  great  period  of  folding  and  dynamometamorphism  in  the 
schists. 

SUMMARY  AIN^D   GEKERAI.  APPLICATION. 

In  order  to  bring  this  hypothesis  of  overhead  stoping,  abyssal 
assimilation,  and  differentiation  into  relation  with  the  general  prob- 
lem of  the  -plutonic  rocks,  it  will  be  expedient  to  recapitulate  (I)  the 
essential  facts  of  observation  in  the  Ascutney  area,  (II)  the  results 
of  experimental  investigation  on  the  specific  gravity  of  the  Ascutney 
rocks  and  on  silicate  magmas,  and  (III)  the  conclusions  won  from  the 
correlation  of  both  groups  of  considerations. 

(I)  The  Ascutney  irruptive  bodies  exhibit  the  following  character- 
istics: 

A  series  of  true  stocks  ranging  from  the  oldest,  most  basic,  and 
least  alkaline  to  the  highly  alkaline,  youngest,  and  most  acid,  followed 
and  accompanied  by  groups  of  aplitic  and  lamprophyric  dikes. 

Two  of  the  stocks  (Basic  stock  and  Main  stock)  characterized  by  a 
noteworthy  heterogeneity;  the  other  three  by  just  as  striking  homo- 
geneity. 

An  almost  entire  lack  of  sympathy  between  the  structural  planes 
in  the  country  rocks  and  the  form  of  each  intrusive  body. 

Conclusive  evidence  that  the  different  magmatic  chambers  were  not 
prepared  by  circumferential  faulting. 

In  each  stock  a  decided  lack  of  any  enrichment  of  the  endomorphic 
zone  by  substance  dissolved  from  the  invaded  formations;  a  general 
freedom  from  foreign  inclusions  in  the  interior,  with  a  characteristic 
abundance  of  angular  inclosures  near  the  contacts;  an  exceedingly 
sharp  line  of  contact  with  the  country  rocks;  equally  sharp  contacts 
of  the  foreign  fragments  and  their  respective  hosts;  lack  of  direct 
sympathy  between  the  composition  of  the  intrusive  stocks  and  of  their 
respective  country  rocks. 

The  existence  of  many  long  and  narrow,  apophysal  offshoots  from 
each  stock,  betokening  their  high  fluidity  at  the  time  of  intrusion. 

The  presence  of  many  basic  segregations  in  four  out  the  five  stocks. 

The  mineralogical  and  chemical  characters  of  the  stock  rocks 
which,  compared  among  themselves  and  with  the  rocks  of  other 
petrographical  provinces,  compel  belief  in  some  kind  of  differenti- 
ation of  the  Ascutney  igneous  bodies  from  a  common  magma. 

(II)  The  experiments  of  Barus,  Delesse,  Daubree,  Doelter,  Oetling, 
Morozewicz  and  others  have  shown — 

That  representative  natural  or  artificial  silicate  mixtures  at  ordinary 


DALY.]  SUMMAEY.  109 

atmosplieric  pressure  become  thinly  molten  at  a  temperature  only 
slightly  above  that  of  solidification. 

That,  in  every  instance,  a  great  increase  of  volume  characterizes 
the  change  from  the  solid  to  the  molten  state. 

That  the  corresponding  difference  of  density  is,  no  doubt,  essentially 
preserved  under  plutonic  conditions. 

That  the  chief  rock-forming  minerals  are  soluble  in  all  of  the  melted 
silicate  mixtures  yet  investigated  and  at  the  temperatures  ruling  when 
those  mixtures  are  thinly  molten. 

That  pressure  aids  the  solubility  indirectly  by  retaining  water  and 
other  mineralizers  in  the  magma,  but  retards  it,  probably  in  much 
less  degree,  by  raising  the  temperature  of  fusion  for  silicate  minerals. 

That  there  is  evidence  of  differentiation  in  molten  silicate  magmas 
by  gravitative  effect. 

Numerous  specific  gravity  determinations  on  the  solid  Ascutney 
rocks  show  that  the  lightest  of  these  would,  under  the  same  conditions 
of  pressure  as  the  densest  of  the  magmas  (that  of  the  Basic  stock), 
sink  on  immersion  in  that  magma. 

(Ill)  The  conclusions  necessitated,  it  is  believed,  by  these  facts  are : 

1.  That  the  various  chambers  now  occupied  by  the  igneous  bodies 
were  not  opened  by  bodily  movements  in  the  earth's  crust,  but  by 
some  kind  of  assimilation  of  the  invaded  formations. 

2.  That  this  assimilation  did  not  take  place,  except  in  subordinate 
degree,  by  caustic  solution  on  the  main  contacts. 

3.  That,  even  in  its  relatively  inactive  state  near  the  moment  of 
final  consolidation,  each  magma  was  capable  of  rifting  off  numerous 
large  and  small  blocks  from  the  walls  wdth  which  it  came  in  contact — 
blocks  now  visible  because  the  magma  was  then  so  toughly  viscous  as 
to  suj^port  them  in  suspension. 

4.  That  during  the  much  longer  period  of  high  fluidity  each  magma 
was  capable  of  still  more  powerful  rifting  action. 

5.  That  throughout  that  period  there  must  have  prevailed  a  more  or 
less  steady  rain  of  the  rifted  blocks  downward  into  the  lower  depths 
of  the  magma  and  a  corresponding  enlargement  of  the  magma  cham- 
ber, the  size  of  which  would  depend  on  the  time  during  which  the 
action  continued;  independent  testimony  may  be  had  of  the  high 
probability  that  the  time  taken  in  all  plutonic  intrusion  is  very  great. 

6.  That  in  the  abyssal  region  the  blocks  must  undergo  active  solu- 
tion by  the  magma,  which  would  thus  become  mixed  and  gradually 
more  complex. 

7.  That  some  compensation  for  the  increased  volume  of  the  rock 
digested  must  be  made — suggesting  either  surface  extrusion  from 
another  part  of  the  same  magma  basin  or  secular  upheaval  of  the 
earth's  crust  above  the  basin. 

8.  That  the  original  magma  was  at  least  as  basic  as  the  gabbroitic 
l)hase  of  the  oldest  stock. 


110  GEOLOGY    OF    ASCUTNEY   MOUNTAllST,   VEEMONT.         [bull.209. 

9.  That  there  would  be  a  tendency  for  the  mixed  magma  to  become 
more  and  more  acid  by  reason,  of  the  assimilation  of  the  schistose  ter- 
ranes. 

10.  That  this  magma  would  be  expected  to  difCerentiate  by  slow 
gravitative  action,  through  which  the  lighter,  more  acid  submagm.as 
would  float  on  the  heavier  basic  residues. 

11.  That  such  differentiation  must  be  supplemented  by  other  causes, 
real  and  universal,  though  at  present  ill  understood,  leading  to  a 
comparatively  definite  splitting  of  the  main  magma;  thus  homogeneous 
rock  bodies  would  be  produced  similar  to  those  in  other  parts  of  eastern 
North  America  and  elsewhere. 

12.  That  the  Ascutney  stocks  are  the  crystallized  product  of  such 
differentiation  from  an  ever-changing  magma  constantly  enriched  by 
assimilation. 

13.  That  the  series  of  petrogenic  events  at  Ascutney  constitute  a 
cycle  that  might  be  repeated  either  as  a  whole  or  in  part  within  the 
same  area. 

14.  That  the  later  basic  dikes  may  be  explained  as  the  beginning  of 
a  second  petrogenic  cycle,  or  as  the  basic  poles  of  a  secondary 
differentiation. 

Now,  the  facts  of  field  observation  at  Ascutney  Mountain,  with  two 
possible  exceptions,  correspond  to  possible  characteristics  of  most  of 
the  granitic  intrusions  of  the  world.  The  heterogeneity  of  the  Basic 
stock  and  of  the  Main  stock  is  doubtless  of  a  higher  order,  and  the 
basic  segregations  in  the  latter  are  more  numerous  than  in  the  normal 
granitic  mass.  Yet  these  contrasts  may  be  largely  explained  by  the 
action  of  secondary  differentiation.  The  experimental  results  of 
investigation  on  melted  silicate  mixtures  are  manifestly  capable  of 
general  application.  There  is,  accordingly,  reason  to  believe  that 
the  hypothesis  summarized  in  the  list  of  conclusions  concerning  the 
Ascutney  eruptives  may  be  applied  to  most  stocks  and  batholiths. 

THE  UIS^IVEESAIj  EARTH  INIAGMA. 

If  this  hypothesis  be  accepted  for  stocks  and  batholiths  generally, 
and  if  dikes,  sheets,  and  laccolithic  intrusions  (including  all  such  as 
have  been  conditioned  by  the  action  of  hydrostatic  pressure  on  a 
magma  entering  spaces  opened  by  bodily  crustal  movements)  are  the 
results  of  the  eruption  of  submagmas  differentiated  from  the  deeper- . 
lying  and  greater  magma  produced  bj^  the  incorporation  of  invaded 
formations,  the  further  inquiry  as  to  the  original  composition  of  such 
assimilating  magma  thus  becomes  a  matter  of  special  interest.  The 
required  space  can  not  here  be  taken  for  a  full  discussion  of  this 
question,  even  if  only  the  limited  number  of  facts  now  known  con- 
cerning the  subject  were  given  full  statement.  Special  dilfidence 
may  be  felt  in  approaching  this  most  difficult  theme.  Yet  certain 
preliminary  considerations  are  offered,  primarily  those  which,  in  the 
opinion  of  the  writer,  do  not  at  the  present  time  receive  the  full  share 


DALY.]  THE    UNIVERSAL    EAKTH    MAGMA.  Ill 

of  attention  that  they  should  have  in  the  problem  of  the  earth's 
interior;  taken  together,  they  seem  to  form,  in  a  measure,  a  test  of 
the  foregoing  hypothesis. 

The  evidence  is  accumulating  that  the  normal  order  for  the  erup- 
tion of  plutonic  rocks  is  that  of  from  most  basic  to  most  acid.  That 
the  same  order  maj^  be  preserved,  on  the  large  scale,  in  extrusions  of 
lava  at  volcanic  cones  is  illustrated  by  Sir  Archibald  Geikie  in  his 
treatise  on  The  Ancient  Volcanoes  of  Great  Britain. «  It  seems 
established,  moreover,  that  the  oldest  eruptive  in  the  majority  of 
petrographical  provinces  approximates  a  gabbro  or  basalt  in  compo- 
sition. Yet  the  oldest  intrusive,  by  the  foregoing  hypothesis,  is  that 
one  which  should  most  nearly  represent  the  original  magma,  modi- 
fied as  the  latter  tends  to  become  by  the  assimilation  of  the  more 
siliceous  crystalline  schists  and  sedimentary  terranes. 

Again,  in  those  conduits  where  escape  of  igneous  rock  from  the 
earth's  interior  to  the  surface  takes  place  to  such  an  extent  as  to 
build  large  volcanoes,  we  should  expect  the  sequence  of  eruption  to 
be  completed  by  an  effusion  of  lava  more  nearly  representing  the 
original  magma  than  the  antecedent  flows.  This  for  the  reasons, 
first,  that  assimilation  (deep-seated  digestion  of  the  overlying  crust)  in 
the  immediate  vicinity  of  the  vent  would,  in  that  late  stage  in  the 
development  of  the  volcano,  have  progressed  so  far  as  to  have 
enlarged  the  conduit  to  a  size  suitable  to  the  large  cone;  secondly, 
that  the  vent  would  by  the  long  continuance  of  the  volcanic  activity 
have  become  freed  from  the  products  of  such  digestion,  and,  thirdly, 
that  the  latest  flows  would  be  derived  from  the  original  magma  prac- 
tically unaffected  hy  assimilation.  Now,  it  is  a  significant  fact  that 
the  latest  extrusive  product  of  our  greatest  volcanoes,  such  as  Etna, 
Fusiyama,  Chimborazo,  Cotopaxi,  etc.,  is  without  known  exception, 
either  basalt  or  andesite.  The  unnumbered  lofty  volcanoes  which 
spring  from  the  floor  of  the  deep  Pacific  and  Indian  oceans  are,  with 
but  few  exceptions,  capped  with  basalt  or  andesite.  Indeed,  such 
basic  lava  seems  to  be  the  only  igneous  rock  exposed  in  oceanic  areas 
making  up  at  least  one  half  of  the  whole  surface  of  the  globe. 

Not  less  important  is  the  equally  indisputable  fact  that  the  great 
fissure  eruptions  of  the  globe  give  birth  to  only  one  kind  of  lava, 
again  basaltic.  The  familiar  examples  in  Iceland,  Northwestern 
Europe,  India,  the  Northwestern  United  States  of  America,  and  the 
Hawaiian  Archipelago,  tell  no  uncertain  story  concerning  the  nature 
of  the  vast  reservoir  from  which  they  have  derived  their  enormous 
volumes  of  lava.  The  more  acid  flows  which  occur  in  any  one  of 
these  regions  are  insignificant  in  bulk  when  compared  to  the  total  basic 
output.  The  former  could  be  explained,  in  accordance  with  the  pres- 
ent hypothesis,  as  the  product  of  differentiation  acting  on  the  uni- 
versal magma  influenced  by  the  assimilation  of  the  continental  rocks, 
which  are  characteristically  more  acid  than  that  magma.     Further, 


a  Vol.  II,  1897,  p.  477. 


112  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,    VERMONT.  [bull.209. 

we  should  expect  assimilation  to  be  less  active  in  determining  the 
composition  of  fissure  eruptives  than  in  preparing  the  secondary 
magmas  erupted  in  volcanic  cones  or  injected  in  the  intrusive  form. 
From  the  nature  of  the  geological  dynamics  rendering  possible  the 
rapid  expulsion  of  the  voluminous  flows  at  great  fissures,  it  is  clear 
that  the  corresponding  magma  had,  in  each  case,  relatively  easy 
access  to  the  earth's  surface,  and  had  not  to  work  its  way  through  the 
crust.  The  plateau  lavas  accordingly  merit  particular  notice  in  the 
search  for  the  general  earth  magma.  Too  little  attention  has  been 
paid  to  the  volume,  relative  abundance,  and  geological  occurrence  of 
the  different  eruptive  types  in  the  extant  discussions  of  the  origin  of 
igneous  rocks.  Those  questions  must  always  be  of  prime  importance 
in  deciding  on  the  question  of  assimilation. 

For  different  reasons,  excepting  that  derived  from  the  enormously 
greater  abundance  of  basaltic  lavas  on  the  earth,  Dutton  came  to  this 
same  conclusion  as  to  the  nature  of  the  "primordial  matter."  lie  has 
rightly  emphasized  the  importance  of  the  fact  that  basalt  is  a  "syn- 
thetic or  comprehensive  type  of  rock."  His  theory  of  the  derivation 
of  other  igneous  rocks  by  simple  fusion  of  sedimentary  formations, 
derived  in  their  turn  by  atmospheric  agencies  from  this  ' '  primordial 
matter,"  takes  insufficient  account  of  the  facts  of  differentiation 
learned  since  1880.  Yet  his  theory  has  a  suggestive  relation  to  the 
one  proposed  in  these  pages. '^^ 

Thus,  partly  by  the  induction  of  known  facts,  partly  by  the  deduc- 
tion of  certain  conclusions  which  are  explanatory  of  a  considerable 
number  of  related  phenomena,  we  have  been  led  to  the  view  that  there 
is,  all  ]-ound  the  earth  and  not  far  from  its  present  surface,  a  single 
fundamental  magma  of  a  composition  allied  to  basalt.  This  magma 
must  probably  be  regarded  as  molten  only  potentially  and  to  uncer- 
tain depth  bj'  the  local  relief  of  pressure.  It  has  been  implied  that 
all  other  rocks  may  have  been  indirectly  derived  from  such  a  magma, 
though  the  possibility  is  not  excluded  that  part  of  the  normal  conti- 
nental intrusive  (acid-alkaline)  rocks  may  form  the  more  or  less  pure 
equivalent  of  primal  matter  differentiated  at  the  surface  of  the  origi- 
nal crust  of  the  earth.  It  is,  of  course,  evident  that  we  are  now  face 
to  face  with  other  principal  earth  problems,  most  of  which  are  nothing 
more  nor  less  than  true  riddles.  The  nature  of  the  earth's  original 
crust,  the  antiquity  of  the  ocean  basins,  the  duration  and  geological 
historj^  of  the  Archean  era  during  which  most  of  the  siliceous  material 
of  the  crust  was  prepared  in  nearly  its  present  form,  the  origin  of  the 
crystalline  schists,  the  preponderance  of  ]3otasli  among  the  alkalies  of 
continental  formations,  the  explanation  of  the  high  soda  content  of  sea 
water,  are  among  those  problems  bearing  on  the  hypothesis.  It  can 
only  be  said  that  the  writer  has  not  yet  met  with  insurmountable  objec- 
tions to  the  hypothesis  in  the  partial  solutions  now  attained  for  them. 

a  Of.  C.E.  Dutton,  The  High  Plateaus  of  Utah:  U.  S.  Geol.  and  Geog.  Surv.  Rocky  .Mountain 
Region,  Washington,  1880,  p.  125  et  seq. 


DALY.]  THE    UTSriVEESAL    EARTH    MAGMA.  113 

The  probability  that  the  combined  variety  and  tyi)e  constancy  of 
the  continental  igneous  rocks  are  due  to  both  abyssal  assimilation 
and  magmatic  differentiation  is'  taught  not  only  by  a  detailed  study 
of  a  small  area  like  Ascutney  Mountain,  but  as  well  bj^  a  review  of 
the  earth's  igneous  output  as  a  whole.  Perhaps  the  hypothesis  founded 
on  this  conviction  may  do  something  toward  removing  the  difficulty 
that  is  felt  by  most  students  of  igneous  rocks;  it  is  the  dilemma  once 
well  described  to  the  writer  by  a  leading  petrologist:  "As  a  geologist, 
one  must  believe  in  assimilation;  as  a  petrographer,  he  must  declare 
against  it." 

Bull.  209—03 8 


APPENDIX. 

TABLES,  LIST  OF  SPECIMENS,  ETC. 

Table  XIII. — Mineralogical  and  structural  constitution  of  the  Ascutney  eruptives. 


Essential  feldspars. 

Other  essential 
constituents. 

Accessory. 

Structure. 

Camptonite 

Basic  labra- 

Hornblende 

Titaniferous 

Panidiomor- 

dorite    (Abj 

Augite. 

magnetite. 

phicporphy- 

Ans). 

Pyrite. 

Apatite. 

Decomposi- 
tion    prod- 
ucts: Chlo- 
rite,    epi- 
dote,    cal- 
cite,   sec- 
o  n  d  a  r  y 
quartz, 
kaolin. 

ritic. 

Augite-gabbro  _ 

do 

Augite. 

Biotite. 

Hornblende. 

Pyrite. 

Ilmenite. 

Titanite. 

Apatite. 

Hypidiomor- 
phic   granu- 
lar. 

Diabase 

do 

do 

Titaniferous 
magnetite. 

Ophitic. 

Pyrite. 

Apatite. 

Decomposi- 

tion    prod- 

ucts: Chlo- 

rite,    epi- 

dote,   cal- 

cite,  sec- 
o  n  d  a  r  y 
quartz, 
kaolin. 

115 


116 


GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT. 


[BULL.  209, 


Table  XIII. — 3Iineralogical  and  structural  constitution  of  the  Ascutney  erup- 

tives — Continued . 


Essential  feldspars. 

other  essential 
constituents. 

Accessory. 

Structure. 

Hornblende- 

Basic  labra- 

Hornblende. 

Ilmenite. 

Hypidiomor- 

biotite-an- 

dorite     (Abj 

Biotite. 

Pyrite. 

phic   granu- 

gite gabbro. 

Ans). 

Augite. 

Titanite. 
Apatite. 

lar. 

Biotite  -  h  o  r  n  - 

Av.  basic  oli- 

Biotite. 

Quartz. 

Do. 

blend  e-dio- 

goclase  (Aba 

Hornblende. 

Ilmenite. 

rite. 

AnJ. 

Pyrite. 
Apatite. 
Titanite. 
Zircon. 

Biotite -augi  te- 

do 

Biotite. 

Quartz. 

Do. 

hornblende- 

Atigite. 

Ilmenite. 

diorite. 

Hornblende. 

Pyrite. 
Apatite. 
Titanite. 
Zircon, 

Essexite 

Andesine  (Abg 

Hornblende. 

Quartz. 

Do, 

Ang). 

Biotite. 

Augite. 

Micropertliite. 

Ilmenite. 

Orthoclase. 

Apatite. 
Titanite. 
Zircon. 

Monzonite 

Microperthite. 

Hornblende. 

.  Quartz. 

Do, 

Ortlioclase. 

Augite. 

Titaniferous 

Labradorite 

Biotite. 

magnetite. 

(AbiAni). 

Apatite. 

Pyrite. 

Zircon. 

Windsorite 

Micropertliite. 
Orthoclase. 

Biotite. 

Quartz. 
AiTgite,  horn- 

Do. 

Basic    oligo- 

clase    (Aba 
Ani). 

blende. 
Ilmenite. 
Apatite. 
Zircon. 
Titanite? 

DALY.] 


APPENDIX. 


117 


Table  XIII. — Mineralogical  and  structtiral  constiUition  of  the  Ascutney  erup- 

tives — Continued. 


Essential  feldspars. 

other  essential 
constituents. 

Accessory. 

Structure. 

Pulaskite 

Microperthite. 

Biotite 

Titaniferous 

Hypidiom  Or- 

Orthoclase. 

magnetite. 

Quartz. 

Titanite. 

Hornblende. 

Augite. 

Apatite. 

Zircon. 

phic   granu- 
lar. 

Nordmarkite  _. 

Microperthite 

Hornblende. 

Quartz. 

Hypidiom  Or- 

(cryptoper- 

Biotite. 

(Allanite.) 

phic  granu- 

thite). 

Augite. 

lar. 

Orthoclase. 

Quartz. 

Tit  anif  erous 

Porphyritic. 

Microcline. 

magnetite. 

Trachytic. 

Acid  oligoclase. 

Apatite. 

Pyrite. 

Zircon. 

Monazite. 
Garnet. 

Biotite-granite  _ 

Microperthite. 

Biotite. 

Magnetite. 

Porphyritic. 

Orthoclase. 

Quartz. 

Titanite. 

Microcline. 

Apatite. 

Acid  oligoclase. 

Zircon. 

Paisanite 

Microperthite. 

Quartz. 

Titanite. 

Panallotrio- 

Soda -ortho- 

Hornblende. 

Ilmenite. 

morphic  por- 

clase. 

Pyrite. 
Zircon. 

Apatite. 

phyritic. 

Muscovite  -  aj)- 

Orthoclase. 

Quartz. 

Microperthite. 

Panallotrio- 

lite. 

Albite. 

Muscovite. 

niorphic. 

118 


GEOLOGY    OF    ASCUTNEY    MOUNTAIN,   VERMONT. 


[BULL.  209. 


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DALY.] 


APPENDIX. 


119 


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120  GEOLOGY    OF    ASCUTNEY    MOUNTAIN,    VERMONT.  [bull.209. 

Table  XV. — List  of  the  more  important  specimens  studied. 

No.    la  and  lb.  Basic  segregation  in  biotite-granite. 

2.  Biotite-granite. 

5.  Metamorphosed  limestone  of  contact-zone,  bearing  grossularite. 
24.  Sericitic  quartzite. 
32.  Biotite-hornblende-diorite. 
34.  G-ranite;  phase  h  of  Main  syenite  stock. 
36.  Breccia  of  Little  Ascutney. 

42.  Green  nor dmarkite;  Main  syenite  stock  (phase/). 
57.  Camptonite  dike;  Little  Ascntney. 

59.  Microperthite-bearing  hornblende-biotite-diorite, 
59a.  Basic  segregation  in  59. 

60.  Paisanite;  Little  Ascntney. 

61.  Hornblende-biotite-angite-gabbro. 

62.  Pulaskite;  Pierson  Peak. 

66.  Basic  segregation  in  porphyritic  phase  g  of  Main  syenite  stock. 
74.  Angite-camptonite. 

76.  Nordmarkite-porphyry;  Little  Ascutney. 

77.  '"  Windsorite""  dike;  Little  Ascutney. 

100.  Metamorphosed  limestone  of  contact  zone,  bearing  epidote,  etc. 

105.  Endomorphic  zone  of  biotite-granite. 

106.  Miarolitic  phase  of  105. 

111.  Monzonite;  phase  i  of  Main  syenite  stock. 

113.  Segregation  in  biotite-granite. 

114.  Unaltered  siliceous  limestone. 

115.  Pori^hyritic  phase  g  of  Main  syenite  stock. 
120.  Diabase  dike. 

122-136,  inclusive.  Metamorphosed  phyllite  of  contact  zone  about  the  Main 

stock. 
139.  Paisanite  of  Main  stock. 
141.  Basic  segregation  in  139. 

145a.  Augite-biotite-diorite  dike;  Little  Ascutney. 
147.  Augite-biotite-hornblende-diorite;  Little  Ascutney. 
175.  Altered  aplite. 
184.  Augite-biotite-diorite. 

191.  Muscovite-aplite. 

192.  Augite-gabbro. 


INDEX 


Page. 

Alkaline  granite,  occiirrence  of. -  -        67 

Alkaline  biotite-granite,  basic  segrega- 
tions in - 83-84 

endomorphic  zone  in  _  _ 84-85 

occurrence  and  character  of 79-85 

Aplitic  dikes,  occurrence  of 70-77 

Ascutney  Mountain,  drainage  of 10-13 

general  form  and  character  of 13 

glaciation  of 13-13 

map  and  section  of 70 

topographic  features  of -  -  -    8-10 

view  of 7 

Augite-biotite-diorite,  occurrence  of 45 

Augite-gabbro,  occurrence  of  _  _ -  -        43 

Augite-hornblende-syenite,  analysis  of  -  -        47 

Barus,  Carl,  cited 95,96,101 

Basic  segregations,  occurrence  and  char- 
acter of  _ _ 43-44 

Biotite-augite-hornblehde-diorite,  occur- 
rence of  _ 40 

Biotite-diorite,  occurrence  of 45 

Biotite-granite,  analysis  of 84 

basic  segregations  in  _ 83-85 

analysis  of 84 

endomorphic  zone  in_ 84-85 

mineral  composition  of 83 

molecular  proportions  of 81 

occurrence  and  character  of 79-85 

Biotite-hornblende-diorite,  occurrence  of       40 

Biotite-nordmarkite,  occurrence  of  - 70 

Bischof,  cited -      101 

Breccia  masses,  Little  Ascutney  Moun- 
tain   _. 77-79 

Brogger,  W.  C,  analyses  compiled  by 41, 

53,54,59,66,75,84,87 

cited 85,93,99,103,106 

Camptonite,  analyses  of 87 

occuri-ence  and  character  of  _  _ 86-87 

Chrustschoff,  cited 65 

Cossa,  cited. 95 

Oushing,  H.  P.,  analysis  by  _ 59 

cited _ 53,89 

Dana,  J.  D.,  cited_ - 13,103 

Delesse,  cited __ 95,97,103 

Diabase,  analysis  of- --        88 

Diabase  dikes,  occurrence  and  character 

of _._  87-88 

Diorite,  analyses  of. 41,66 

occurrence  and  character  of 38-44 

Dioritic  dikes,  features  of 44-45 

Dikes,  occurrence  of - 69, 70-77, 86 

Dittrich,  analysis  by 41 


Page. 

Doelter,  C,  cited 94,98,99,101,103,105-106 

Doja,  M.,  analysis  by 39 

Dresser,  cited 104, 105 

Dutton,  C.  E.,  cited 113 

Earth  magma,  original,  composition  of.  110-113 

Emerson,  B.  K.,  cited 19 

Eruptive  rocks,  abyssal  assimilation  of 

country  rock  by 100-103 

character  and  occiirrence  of 38-89 

methods  of  differentiation  of 104-107 

table  and  correlation  of .  _ 36-37 

Essesite,  analyses  of 41,66 

Gabbro,  occurrence  and  character  of 38-44 

Geikie,  cited _-_ 106 

Gneissic  series  of  rocks,  occurrence  and 

character  of 17-19 

Gouy  and  Chaperon,  cited - .      106 

Graber,  analysis  by.- 39 

Granite,  alkaline,  occurrence  of ;..       67 

basic  segregations  in 83-84 

endomorphic  zone  in -  _  84-85 

occiu'rence  and  character  of 79-85 

Granitite,  analysis  of 84 

Granodiorite,  analysis  of  _ 47 

Grorudite,  analysis  of _        75 

Gulliver,  F.  P.,  acknowledgments  to____.  7 

Harker  and  Marr,  cited.. -. -.       23 

Hawes,  cited 73-74 

Hillebrand,  "W.  F.,  acknowledgments  to  .  7 

analyses  by 15, 37, 39, 

41,44,47,59,  60,66,  75,76,81,83,84,87,88,119 

cited 37 

Hitchcock,  C.  H.,  cited .--  8,15,64 

Hitchcock,  Edward,  cited 17, 19 

quoted 77 

Hornblende-biotite-augite-gabbro,  occur- 
rence of 43 

Hornblende-biotite-diorite,  occurrence  of       43 

thin  section  of,  plate  showing 33 

Hornblende-paisanite,  analysis  of 75 

Hornf els,  plate  showing 33 

Hunt,  T.  S.,  cited 19 

Intrusion  of  the  stocks,  hypotheses  con- 
cerning   90-103 

Intrusive  rocks  of  the  area,  geologic  age 

of.._ -  19-30 

Irruptive  rocks  of  Ascutney  Mountain, 

characteristics  of 108 

date  of  intrusion  of 21 

specific  gravities  of .- 97 

Jaggar,T.A.,  acknowledgments  to 7 

Johnston-Lavis,  cited 99 

131 


122 


INDEX. 


Page. 

Joly,  cited  .- __-        95 

Judd,  cited _ _        48 

Kemp,  J.  F.,  cited 85 

Lacroix,  cited 34,42 

Lagorio,  cited ,    102 

Lamprophyres,  occurrence  and  charac- 
ter of - ___.  85-88 

Lawson,  A.  C,  cited _ 98-103 

quoted 99-100 

Lestivarite,  analyses  of 75 

Limestones,  changes  in 23 

metamorphic  constituents  of 35 

Lindgren,  Waldeniar,  cited  on  composi- 
tion of  granodiorite  _  _ 47 

Little  Ascutney  Mountain,  breccia  masses 

on : -. 77-79 

intrusive  rocks  on,  plan  showing 74 

paisanite  dike  on 73-77 

syenite-porphyry  dike  of 69 

view  of -  -  7 

Metamcjrphic  aureole,  features  of  _  _ 23 

Metamorphism,  effects  of 33-35 

Monzonite,  analysis  of 66 

occurrence  of 68 

Morozewicz,  J.,  cited 28,105 

Mount  Ascutney.    See.  Ascutney  Moun- 
tain. 

Muscovite-aplite,  occurrence  of 73 

Nordmarkite,  analyses  of  _ .  ^ 47, 59 

basic    segregations   in,    plate    show- 
ing   - - .  - 62 

mineral  composition  of 60 

molecular  proportions  of 60 

occurrence  of 53,61,64^69 

Nordmarkite-porphyry,  dikes  of 69 

Oetling,  cited 98,102 

Osann,  analysis  by ^ 75 

Paisanite,  analyses  of 75 

occurrence  of 70-77 

mineral  composition  of 73, 76 

molecular  proper tions  of 76 

thin  section  of,  plate  showing _  _  62, 64 

Phyllite,  plate  showing 16 

metamorphism  of 24-35 

Phyllitic  series  of  rocks,  occurrence  and 

characterof - 14-17 

Pierson  Peak,  location  of 9 

syenite  stock  of.. - 70 

view  of -- 7 

PulasMte,  analysis  of _ 59 

occurrence  of ..- 70 

Pyroclastic  feldspar,  thin  section  of,  plate 

showing 58 


Quartz-diorite,  occurrence  of 38 

Quartz-monzonite,  analysis  of 47 

Quartz  -sericite-schist,  analysis  of  _  _  _ 15 

Quartz-syenite,  analysis  of 59 

Reade,  cited _ 96 

Richardson,  C.  H.,  cited 15,20 

Rosenbusch,  H.,  acknowledgments  to  ...  7 

analysis  by 87 

Schists  of  the  area,  geologic  age  of 19-20 

Smyth,  C.  H.,  analysis  by 47 

Specific  gravity  of  eruptive  rocks,  table 

showing 97 

Specific    gravity  and    temperatures  of 

rocks,  relations  of 95 

Stock  rocks,  intrusions  of,  theories  con- 
cerning   90-100 

Syenite,  allanite  in 55-56 

apatite  in 56 

augite  in _ 54 

basic  segregations  in 64-68 

biotite  in .        55 

dikes  cutting 70-77 

endomorphic  zone  of 08-69 

feldspars  in 50-51 

garnet  in 57 

hornblende  in 53-54 

magnetite  in _  _        56 

monazite  in _ _ 56 

occurrence  and  character  of 48-70 

quartz  in 57 

tarnishing  of,  on  exposure  to  air 51-53 

titanite  in :        56 

zircon  in  _ _ . . . .  58-57 

Syenite-porphyry,  analysis  of 59 

dike  of,  at  Little  Ascutney  Mountain.        69 

Teller  and  von  John,  cited 35 

Temperature    and    specific    gravity    of 

rocks,  relations  of  __ 95 

Turner,  H.  W. ,  analysis  by 47 

VanHise,  C.R.,  cited 103 

quoted  _ _ 77 

Washington,  H.  S.,  analy.sis  by 75 

cited .-.. 89 

Weed  and  Pirsson,  cited 93 

Whittle,  C.  L.,  cited-. 15 

Williams,  G.  H.,  cited 35 

analysis  by - 59 

Windsorite,  analysis  of 47 

definition  of 48 

mineralogical  composition  of   46 

dikes  of 45-48 

Wolff,  J.  E.,  acknowledgments  to 7 

Zirkel,  P.,  cited 95 


o 


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[Bulletin  No.  209.] 

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B  23.  Observations  on  the  junction  between  the  Eastern  sandstone  and  the  Keweenaw  series  on 
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B  33.  Notes  on  geology  of  northern  California,  by  J.  S.  Diller.    1886.    23  pp. 

B  39.  Tlie  upper  beaches  and  deltas  of  Glacial  Lake  Agassiz,  by  Warren  Upham.   1887.   84  pp.,  1  pi. 

B  40.  Changes  in  river  courses  in  Washington  Territory  due  to  glaciation,  by  Bailey  Willis.  1887. 
10  pp.,  4  pis. 

B  45.  The  present  condition  of  knowledge  of  the  geology  of  Texas,  by  Robert  T.  Hill.    1887.    94  pp. 

B  53.  The  geology  of  Nantucket,  by  Nathaniel  Southgate  Shaler.    1889.    55  pp.,  10  pis. 

B  57.  A  geological  reconnaissance  In  southwestern  Kansas,  by  Robert  Hay.    1890.    49  pp.,  2  pis. 

B  58.  The  glacial  boundary  in  western  Pennsylvania,- Ohio,  Kentucky,  Indiana,  and  Illinois,  by 
G.  F.  Wright,  with  introduction  by  T.  C.  Chamberlin.    1890.    112  pp.,  8  pis. 

B  67.  The  relations  of  the  traps  of  the  Newark  system  in  the  New  Jersey  region,  by  N.  H.  Barton. 
1890.    82  pp. 

B  104.  Glaciation  of  the  Yellowstone  Valley  north  of  the  Park,  by  W^  H.  Weed.    1893.    41  pp.,  4  pis. 

B  108.  A  geological  reconnaissance  in  central  Washington,  by  I.  C.  Russell.    1893.    108  pp.,  12  pis. 

B  119.  A  geological  reconnaissance  in  nortliwest  Wyoming,  by  G.  H.  Eldridge.    1894.    72  pp.,  4  pis. 

B  137.  The  geology  of  the  Fort  Riley  Military  Reservation  and  vicinity,  Kansas,  by  Robert  Hay 
1896.    35  pp.,  8  pis. 

B  144.  The  moraines  of  the  Missouri  Coteau  and  their  attendant-deposits,  by  J.  E.  Todd.  1896.  71 
pp.,  21  pis. 

B  158.  The  moraines  of  southeastern  South  Dakota  and  their  attendant  deposits,  by  J.  E.  Todd. 
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B  159.  The  geology  of  eastern  Berkshire  County,  Massachusetts,  by  B.  K.  Emerson.  1899.  139  pp., 
9  pis. 

B  165.  Contributions  to  the  geology  of  Maine,  by  H.  S.  Williams  and  H.  E.  Gregory.  1900.  212  pp., 
14  pk. 

WS  70.  Geology  and  water  resources  of  the  Patrick  and  Goshen  Hole  quadrangles  in  eastern  Wyom- 
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B  199.  Geology  and  water  resources  of  the  Snake  River  Plains  of  Idaho,  by  I.  C.  Russell.  1902.  192 
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I 


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of  the  geology  of  southeastern  Alaska,  by  Alfred  Hulse  Brooks.    1902.    120  pp.,  2  pis. 

PP  2.  Reconnaissance  of  the  northwestern  portion  of  Seward  Peninsula,  Alaska,  by  A.  J.  Collier. 
1902.    70  pp.,  11  pis. 

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1902.    167  pp.,  19  pis. 

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PP  11.  Clays  of  the  United  States  east  of  the  Mississippi  River,  by  Heinrich  Ries.  1903.  —  pp., 
9  pis. 

PP  12.  Geology  of  the  Globe  copper  district,  Arizona,  by  F.  L.  Ransome.    1903.    168  pp.,  27  pis. 

PP  13.  Drainage  modifications  in  southeastern  Ohio  and  adjacent  parts  of  West  Virginia  and  Ken- 
tucky, by  W.  G.  Tight.    1903.    —  pp.,  17  pis. 

B  208.  Descriptive  geology  of  Nevada  south  of  the  fortieth  parallel  and  adjacent  portions  of  Cali- 
fornia, by  J.  E.  Spurr.    1903.    —  pp.,  8  pis. 

B  209.  The  geology  of  Aseutney  Mountain,  Vermont,  by  R.  A.  Daly.    1903.    122  pp.,  7  pis. 

SERIES   D,    PETROGRAPHY    AND    MINERALOGY. 

B  1.  On  hypersthene-andesite  and  on  triclinia  pyroxene  in  augitic  rocks,  by  Whitman  Cross,  -with 
a  geological  sketch  of  Buffalo  Peaks,  Colorado,  by  S.  F.  Emjnous.    lSg3.    42  pp.,  2  pis. 

B  8.  On  secondary  enlargements  of  mineral  fragments  in  certain  rocks,  by  R.  D.  Irving  and  C.  R. 
Van  Hise.    1884.    56  pp.,  6  pis. 

B  12.  A  crystallographic  study  of  the  thinolite  of  Lake  Lahontan,  by  E.  S.  Dana.    1884.    34  pp.,  3  pis. 

B  17.  On  the  development  of  crystallization  in  the  igneous  rocks  of  Washoe,  Nev.,  with  notes  on 
the  geology  of  the  district,  by  Arnold  Hague  and  J.  P.  Iddings-.    1885.    44  pp. 

B  20.  Contributions  to  the  mineralogy  of  the  Rocky  Mountains,  by  Whitman  Cross  and  W.  F.  Hille- 
brand.    1885.    114  pp.,  1  pi. 

B  28.  The  gabbros  and  associated  hornblende  rocks  occurring  in  the  neighborhood  of  Baltimore, 
Md.,  by  G.  H.  Williams.    1886.    78  pp.,  4  pis. 

B  38.  Peridotite  of  Elliott  County,  Ky.,  by  J.  S.  Diller.    1887.    31  pp.,  1  pi. 

B  59.  The  gabbros  and,  associated  rocks  in  Delaware,  by  F.  D.  Chester.    1890.    45  pp.,  1  pi. 

B  61.  Contributions  to  the  mineralogy  of  the  Pacific  coast,  by  W.  H.  Melville  and  Waldemar  Lind- 
gren.    1890.    40  pp.,  3  pis. 

B  62.  The  greenstone-schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan;  a  contri- 
bution to  the  subject  of  dynamic  metamorphism  in  eruptive  rocks,  by  G.  H.Williams;  with  introduc- 
tion by  R.  D.  Irving.    1890.    241  pp.,  16  pis. 

B  66.  On  a  group  of  volcanic  rocks  from  the  Te wan  Mountains,  New  Mexico,  and  on  the  occur- 
rence of  primary  quartz  in  certain  basalts,  by  J.  P.  Iddings.    1890.    34  pp. 

B  74.  The  minerals  of  North  Carolina,  by  F.  A.  Genth.    1891.    119  pp. 

B  79.  A  late  volcanic  eruption  in  northern  California  and  its  peculiar  lava,  by  J.  S.  Diller.  1891. 
33  pp.,  17  pis. 

B  89.  Some  lava  flows  of  the  western  slope  of  the  Sierra  Nevada,  California,  by  F.  L.  Ransome. 
1898.    74  pp.,  11  pis. 

B  107.  The  trap  dikes  of  the  Lake  Champlain  region,  by  J.  F.  Kemp  and  V.  F.  Masters.  1893.  62 
pp.,  4  pis. 

B  109.  The  eruptive  and  sedimentary  rocks  on  Pigeon  Point,  Minnesota,  and  their  contact  phe- 
nomena, by  W,  S.  Bayley.    1893.    121  pp.,  16  pis. 

B  126.  A  mineralogical  lexicon  of  Franklin,  Hampshire,  and  Hampden  counties,  Mass.,  by  B.  K. 
Emerson.    1895.    180  pp.,  1  pi. 

B  136.  Volcanic  rocks  of  South  Mountain,  Pennsylvania,  by  Florence  Bascom.  1896.  124  pp., 
28  pis. 

B  150.  The  educational  series  of  rock  specimens  collected  and  distributed  by  the  United  States 
Geological  Survey,  by  J.  S.  Diller.    1898.    400  pp.,  47  pis. 

B  157.  The  gneisses,  gabbro-schists,  and  associated  rocks  of  southwestern  Minnesota,  by  C.  W. 
Hall;    1899.    160  pp.,  27  pis. 

PP  3.  Geology  and  petrography  of  Crater  Lake  National  Park,  by  J.  S.  Diller  and  H.  B.  Patton. 
1902.    167  pp.,  19  pis. 

B  209.  The  geology  of  Aseutney  Mountain,  Vermont,  by  R.  A.  Daly.    1903.    122  pp.,  7  pis. 

Correspondence  should  be  addressed  to 

The  Director, 
United  States  Geological  Survey, 

Washington,  D.  C. 
March,  1903. 


LIBRARY  CATALOGUE  SLIPS. 

[Take  this  leaf  out  and  paste  the  sepai'ated  titles  upon  three  of  your  catalogue 
cards.  The  first  and  second  titles  need  no  addition;  over  the  third  write  that 
subject  under  which  you  would  place  the  book  in  your  library.] 


United  States.     Department  of  the  interior.     ( U.  S.  Geological 
survey. ) 

Bulletin  No.  209    Series  \  ^'  Descriptive  geology,  28  1 

(  D,  Petrography  and  mmeraiogy,  22  I 
Department  of  the  interior  |  United  States  geological  survey  | 
Charles  D.  Walcott,  director  ]  —  1  The  |  geology  of  Ascntney 
Mountain,  Vermont  |   by   |    Reginald   Aid  worth   Daly  |    [Vi- 
gnette] I 

Washington  |  government  jprinting  office  |  1903 
8°.    133  pp.,  7  pis. 


Daly  (Reginald  Aid  worth) 

BulletinNo.209    Series -j  ^'^^"P*^^  ^^^^l^^^'"^  ,  \ 

i  D,  Petrography  and  mineralogy,  22  I 

Department  of  the  interior  |  United  States  geological  survey  | 
Charles  D.  Walcott,  director  |  —  |  The  ]  geology  of  Ascutney 
Mountain,   Vermont   |    by   |    Reginald  Aid  worth  Daly  |    [Vi- 
gnette] I 

Washington  |  government  i)riuting  office  |  1903 

8°.    123  pp.,  7  pis. 


Bulletin  No.  209    Series  j  ^' ST"''*\'^'*'\''^'^-''^  i         oo  I 
(  D,  Petrography  and  mineralogy,  22  I 

Department  of  the  interior  |  United  States  geological  survey  | 
Charles  D.  Walcott,  director  |  —  |  The  |  geology  of  Ascutney 
Mountain,  Vermont    |   by  |    Reginald  Aid  worth   Daly    |    [Vi- 
gnette] I 

Washington  |  government  printing  office  |  1908 

8°.    123  pp.,  7  pis. 

Ill 


BOSTON 


COLLEGE 


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